Well, U have two options.
Going for
1.AMD64 or
2.Intel or AMD Athlon
If U choose 1, U wil b able to enjoy ur DOOM3 to the fullest, boast about it to ur friends, and will b able to upgrade to PCI-Express only by changing the Motherboard.
If U choose 2, then U are a normal guy with a normal PC and U will have to upgrade to 64 bit anyway when U will b thinking ok PCI Express based cards and hence U will have to upgrade ur processor as well as motherboard.
Btw, have a look at AMD 3500(socket 939) too.