Here is some ideas
ATX case - make sure it is ATX
400-500w ATX Power Supply - Make sure the power supply will work with your motherboard. Most are standard
ATX Motherboard - I prefer ASUS because there is no configurations that need to be done
AMB 64bit Processor - Depending on what you can afford the bigger the better. Make sure that it works with your motherboard.
I bought my last motherboard/cpu/memory bundle from MonarchComputers.
http://www.monarchcomputer.com/Merchant2/merchant.mv
1gb of RAM at the minimum of course the more the faster - Make sure it works with your motherboard
120GB or larger Hard Drive - Hard Drives are cheap so get the biggest you can afford
High end graphics card - I got a MSI NX6600GT-VTD128 Geforce 6600GT 128MB 128-bit GDDR3 VIVO AGP 4X/8X Video Card
http://www.newegg.com/Product/Produc...82E16814127150
It plays Battlefield 2 flawlessly.
Sound card - I prefer Creative Labs Soundblaster Audigy 2 or better
DVD-RW Drive
Mouse
Keyboard
Monitor
I think that is it.
My recommendation and I know some will disagree but I would tell you to buy a motherboard/CPU/RAM bundle because you know that they all work together and that is less that you have to worry about.
Just my .02 cents.