Ok that is actually a great 550W PSU you have there. I am going off that you already have a case,hd,and as you said opticals and the PSU. So here is what I came up with. for mobo,ram,cpu,video and tried to keep it close to $500
$179.99 SAPPHIRE 100186L Radeon X1950XT 256MB GDDR3 PCI Express x16 VIVO HDCP Video Card
$112.99 G.SKILL 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory 4-4-4-12
$99.99 GIGABYTE GA-965P-DS3 (rev. 1.3) LGA 775 Intel P965 Express ATX Intel Motherboard
$116.00 Intel Core 2 Duo E4300 Allendale 1.8GHz LGA 775 Processor
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$508.97 TOTAL
That gives you a good motherboard which supports 1333FSB and quadcore chips for future proofing yourself. 2GB of decent 4-4-4-12 memory at DDR2 800 if you want to overclock, which you will, just maybe not this second. But the core2's do overclock well and you will want to if you hang around here long enough. The e4300 core 2 which is cheap but can overclock like a ah heck. And probably the best midrange video card you can get for $180. Anything higher you may as well pony up and get an 8800gts 320MB.
so there you go. my recommendations. If I had $500 in your situation.
Why?... because Mike Rowe said so.