Does atmospheric pressure effect operating temperatures of circuits and/or IC's implemented on a system in space?
It does effect the operating temperature of circuits and ICs implemented in space. It does this by changing the thermal dissipation mechanisms available to the ICs, as pjc50 noted in his answer. For regular applications, you rely almost exclusively on convective cooling cause by air flow. The thermal junction properties listed in power transistor and IC datasheets assume that the device is in air, and heat sink specifications assume the heat sink is in air. Thus the many fins in a regular heat sink - Increasing surface area increases the contact area with air and allows it to pull away more heat. This is completely absent in space (well, there's some air pressure if you're in low Earth Orbit, but the cooling effect drops to negligible levels even there). The problem is solved by using a combination of techniques to cool the circuitry and maintain the electronics within an allowed region. These are generally a combination of active and passive thermal control systems (including heating, not just cooling). Heat sinking is usually conductive, piping away heat to larger areas which can radiate effectively into the dark of space.
Additionally, there is rapid 'thermal cycling' if you're orbiting something between eclipse and non-eclipse periods, and also thermal gradients that are set up between the illuminated and dark sides of the satellites. These require better controlled fabrication processes and materials to avoid things cracking, breaking, and otherwise degrading.
Further, the vacuum itself causes a non temperature related problem in that the materials used may vaporize by a process known as degassing. This could cause wire insulation to fray, ICs to decapsulate, condensation of this vaporized material on optics, weakening of mechanical components, change of dielectric (and therefore RF) properties, change of thermal properties, etc.
Does radiation/gases effect operating temperatures of a circuit and/or IC's in the same scenario above? If so, does the engineer have to take in precautionary measures in the construction of a circuit if the system might be exposed in low orbit of planetary systems?
Radiation does. Gases, well, depends on which gasses. If its a highly ionizable gas, then it could spark easily.
Radiation effects the operation in a way other than in temperature. Radiation directly attacks the ICs and causes faulty operation and even failure of the device. This is also a problem on the Earth, by the way, and high performance computing clusters with thousands of nodes see data corruption and even node failure often enough to make it a serious problem. It would happen with desktops too, except that since you'd only be looking at a single node in isolation the failure rate seems incredibly low and mostly goes unnoticed.
The two principle ways in which radiation effects electronics is by latch-up and SEUs. Latch ups occur when a charged particle gets lodged in a gate. This effectively shorts the gate and causes a high current to flow through it. The fact that the gate is charged to begin with attracts the particles in the first place, and the current keeps it lodged in. If the situation persists, the gate would degrade and in the worst case condition cause the IC itself to fail. The way this is fixed is to cause the power to cycle, which is done by intelligent watchdog and power systems, which is usually enough to dislodge the particle. The second, more common kind, is a single event upset (SEU), where a single bit of memory is flipped because of a passing charged particle. This can cause data corruption, and depending on where the bit is (program counter, for instance), more serious system failure. This is overcome using a method known as triple majority redundancy (TMR), where each bit is stored in three places and is periodically checked (or checked at the time of use). The assumption is that the bit that is damages is unlikely to be damaged in all three copies, since this is a fundamentally random event.
The smaller your feature size (IC fabrication process), the larger the chance of one of these happening. The hotter the IC, the higher the chance of one these happening (although by a small factor).
Radiation hardening is done in the hardware and often IC level by building TMR into the IC itself. In fact, there are space grade processors which even have 3 cores operating in parallel, doing exactly the same thing. At a higher level, redundancy is maintained at a board or package level, and is used in some cases as a fallback and in others in a TMR kind of fashion. The chips are themselves ruggedized, to tolerate more heat, dissipate more heat by radiation, much better controlled processes so there are less 'outliers' which can make radiation's job easier, and often have embedded plates for shielding using a radiation opaque material. This depends on what sort of radiation you're worried about, but usually a plate of tantalum works in low earth orbit.
What kind of precautionary measures are we talking about in GENERAL terms? Are there any good resources out there that explains how circuits can generate heat and tactics used to lower their levels of heat given off?
There are general things that are done which are common even for regular electonics. FMEA / FMECA can help prioritize potential problem areas. Careful design and failure analysis can help identify and eliminate all sources of single point failure (where a single problem can cause catastrophic failure). Additionally, there are other measures taken such as careful selection of material for behaviour in vacuum, radiation, and thermal excursion. The NASA System Engineering handbook had some fairly good explanations, if I remember correctly. Off hand, I can't remember any specific, non-classified source which has the details collected in a single place.