Budget builds can actually be quite nice, as I have found with mine. Best? Not quite, but dang it's nice.
Motherboard: Biostar NF61S-M2 TE / AM2 Socket 50$
CPU: AMD Athlon X2 4850e 2.5GHz 65$
RAM: 2GB (2x 1GB) DDR2-800MHz 40$
GPU: Nvidia GeForce 7900GS 100$
PSU: OCZ Stealth 700Watt 90$ (Arrives Wednesday, old PSU is trash)
HDD: Western Digital 360GB EIDE 65$
Case: Aerocool M40 80$
Total: 490$
I am soon replacing the GPU with a GeForce 9800GTX that will bring the price up to about 540$. (Around March)
It may be a little more than some consider "budget", but most parts have rebates and all parts are good brands.
It works very nicely, although some may hate my 'Cube Case'. Currently it runs all my games from old C&C games (1995) to AH (current) almost perfectly. Games such as new C&C to Crysis run on Medium with acceptable FR.