You want to get $500 out of something which would cost $450ish to buy new? No chance, I'm afraid. It depends where you sell it as well. I'd say maybe more like $350. Not trying to be mean or anything, but second hand computer prices aren't generally that good.
No one cares how much it cost new. I can tell you it's not worth much, maybe $300. The problem is that you have a lot of dying tech in there, such as the LGA775 CPU (dual core even), an old nVidia GPU, and DDR2. Then you also have low quality parts as well, like the POS PSU and HSF.
You could build something new for $500 that would just destroy that build and still have quality parts with an upgrade path. That's the reason you can't just add up what the parts cost. The typical formula is add up the individual parts, multiply by 80%, then subtract 10% for every 3 months it was used. I subtracted 30% for age simply because of the bad quality parts.
MadAdmiral, I think you are being optimistic. I'd say $250, tops. Maybe 40% on CPU, memory, and video card. Zero on everything else: PSU cooler, motherboard, and PSU are junk. And the egg has new 250 GB hard drives for $45.
You might do better breaking it down and selling the parts separately.
Who cares, last I checked the egg had 500GB drives for $55. And that was shipped to. This is the problem with trying to sell a computer. If you look up EXACTLY what each part is, you'll see these nice high prices. But who cares? Why should I spend $174 on a dual core CPU, when I can buy an AMD 945 or 955? Why should I spend $89 on some DDR2 memory when I can buy 4GBs of DDR3 for the same price? As was pointed out, if someone wanted to use those prices they could build something faster/better, and still have a better machine.
I think the better question is would the OP get more by selling the complete system or parting it out on fleaBay. You should watch auctions for the same parts (used, not new) and see what the going price is.
E8400 = $100 if your lucky, more like 90.
GTX260 = $140 (he's not far off on this. I have seen them go for more sometimes.)
4GBs DDR2-800 = $60
mATX S775 board = $40
junk PSU = ??? Good 500Ws go for $40 now. This probably won't even be touched on a tech forum. Same with the CPU cooler. Might be able to sell for $10 a piece or something.
250GB = $25.
Add them up, and your looking around $350 if you sell the parts I listed prices on. The big problem your going to have is thats shipped prices, so knock a bunch off for shipping. This will vary depending on method and if people buy in bulk. Your take home will be closer to $300. The other problem you'll have is actually selling everything. As pointed out, not everyone will want the mATX board or the junk PSU.