Here is what I would suggest:
AMD Athlon 64 X2 6000+ Windsor 3.0 Ghz Dual Core CPU
Gigabyte Motherboard Supports up to 16 GB DDR2 1066 Ram if you ever switch over to a 64 bit OS.
4GB OCZ Platinum Dual Channel Ram
Western Digital 500GB 7200 RPM SATA 3.0 Hard Drive
ASUS GeForce 9600 GSO 384 MB GDDR3 Ram Supports DX10 and OpenGL 2.0, 96 Stream Processors, 1800Mhz Memory Clock and 600Mhz Core Clock
And the above processor comes with a heat sink and fan but if you are looking for something maybe a little bit better or quieter you might try this one
ARTIC Cooling Heatsink and Fan Copper core with copper heat pipes. .4 sone noise level and 3000+ RPMS
Total: $510.94
This is just a suggestion. I know the motherboard does not support SLI; however, you said that you are not worried about insane graphics so I do not think it is something that you would need to worry about. Also, for the HD I only put one 500GB drive, if you wanted to do a RAID 0 or RAID 1 system you can easily add a second drive for another $70 still keeping your total under $600 before shipping and without case/power supply. As for a power supply, whatever brand you go with I would suggest a 500W power supply. Antec and Thermaltake are good and well known brands but you are also going to pay for that reputation. I would suggest taking a look on
http://newegg.com and see what they have to offer as far as 500W power supplies go. Look for ones with a 5 egg rating and a lot of reviews this way you know that you are getting a decent power supply.