If you don't build yourself check out Dell systems. Unless you're running pretty large models I would get a single dual core processor machine with 4 GB or RAM and a 64 bit Operating System. Windows x64 serves me well. Make sure to get plenty of storage and if you're really looking to spend some $$ SAS hard drives for your OS and scratch space would be quite nice. There are certainly bigger and badder firebreathing machines out there but they generate a lot more heat and are noisier so unless you really need a LOT of power something like the above should do just fine.
Good luck,
-Brian