It really depends a lot on what kind of games you play and whether you want to build your own system. I presume you don't have the experience to build your own (which is by far the cheapest means and also allows for upgrades down the road) so you'll have to by from a retailer. From experience the best/cheapest rigs come from Dell. Since your games are not extremely graphics intensive you won't need to spend much. A good dual core, maybe the Core 2 Duo E7300, 3 GB RAM and a low-end videocard like the Geforce 8600GT or ATI 3450 should suffice. You can put these together in a Dell Inspiron build at their website.
I would like to add that the answer beneath mine is nonsense. First, not everyone can just go out and build a computer, oftentimes putting the pieces together is the easiest part. The difficulty is knowing what parts you need (its like a puzzle) not every part works with another and how to troubleshoot when things don't work right at first for they rarely do.
Also, the ranting about which windows you need is ridiculous, all the games you mention, as with any game made after 2003 works perfectly with Vista and directx is not an issue ever, just update when you need to.
DON'T ever use a USB harddrive as your primary drive, they are much more prone to failure and are not designed to be used as extensively as an internal drive.
Actually, please just don't read that answer. I have 15 years of PC building experience and they're obviously incompetent.
If you really want to try building one yourself consult the forums at www.tomshardware.com extensively. There are many people there who will help you every step of the way and they are much more knowledgeable than folks around here.
Also, for those games have these as parts as a minimum.
Dual core processor (AMD 64x2 4000+ or better, or a Intel Core 2 Duo E5400)
PCI express videocard with at least 256MB memory (Nvidia GeForce 8600GT or ATI X1650) DO NOT use AGP cards.
1-2GB RAM for XP or 3GB for Vista