Start with the system requirements that the game has
CPU 2 GHz, Pentium 4
- Memory (RAM)
1 GB
- Graphics
GeForce 6800/Radeon X800 With 128 MB
- Disk Space
4.6 GB
- Optical Drive
DVD-ROM
- Windows Compatibility
2000/ XP
- Mac Compatibility
No
It's pretty difficult to buy a desktop nowadays that doesn't meet the above requirements. The only thing you need to watch out for is that the computer has, or can be upgraded to have a standalone video card. The budget ( just under $200 ) video cards you can buy from nvidia or ati are above and beyond what you'll need for the minimum requirements.
Now if you want to play it with higher settings ( which you probably do ) you can start upgrading the important things... CPU/Memory/Video card/HD. Fallout 3 uses multi-threading by default, so getting an i5 or i7 cpu with multiple cores is nice. If you increase your memory, you can increase the memory buffer settings in the fallout .ini files. A really high end video card from nvidia or ati will let you max out the video settings, and finally, going with a solid state drive instead of an sata drive in your desktop will speed up many of the load screens and open up one of the bottlenecks of most desktop system.
You can also visit http://forums.bethsoft.com/index.php?/forum/36-fal
lout-3-pc-issues/. It's the official forums for fallout3, and that specific link brings you to their PC hardware/software problem forums. You can see what problems people are having with specific pieces of hardware that you might be considering, along with tons of people posting about PCs they are planning on buying or have purchased specifically for the game. If all of the above is more than you really wanted to get into, and you have some cash to throw at the problem, try searching for CyberpowerPC Intel i7 930 Computer (GLX202) - English - Web Only
It's probably a bit more than you need, but covers the minimum requirements easily and has the majority of the upgrades listed above as well.
Good luck with the new PC
