used, built in 2006 during july.
raidmax sagita case. 500w psu
amd 64 x2 4800+ socket am2
asus m2n32 sli deluxe with wifi
2 gig ddr2 800 dual channel latency 5 5 5 18
300 gig harddrive
sony optical drive
bfg 7900gt oc
windows xp 32

What do you think I could get for that pc?

Oh, I would say, maybe about $300. Maybe more, I would love your computer though.

First I'd ask you to double check your system specs, because what you have is incorrect.
Socket AM2 came out in mid 2006.
So if you built this system in mid 2004 it must be Socket 939, and that would be DDR not DDR2, changing the latency of the memory, changing the motherboard...
Better double check your system stats to get a more accurate estimate

 
woops my bad >_<
 
july 06, it was right after am2 came out... i paid out of the bunghole for the mobo and processor

Try 500 or so.  It's not bad by any stretch.  Maybe a little more than 500, you could always negotiate downward.  

i you sell it as a "super duper graphics and speed, will play any of todays game!" you might get 750$ even

 
how should I sell it, I have never done this kinda thing before. Do I use ebay? or what?

Could do ebay....craigslist, only thing I would do is make sure I explicitly explained and made perfectly clear that it was sold "as is"  with NO warranty of any kind.  Really though you might be better to sell it locally so you don't have to worry about it being damaged in shipping.  I'm picturing the cpu heatsink coming off and smashing a video card or something, or rattling around inside the case...may not make for happy buyers.....
 
Also, I would look into some serious ways of erasing my data.  Maybe doing at least 7 passes of writing all zeros onto that hard drive.  Or making the people buy their own hard drive somewhere else.  Just to make sure you protect yourself and any confidential or personal data you may or may not have had on the hard drive.  

1 zero pass will erase any data that could be used for practical purposes. Any further passes are superfluous and a waste of your time.

 
this is a good point. i may not include the harddrive... then i can just use it in my new build :-)

 
 
I would be paranoid! maybe I could like microwave the HDD to make sure everything is gone. or not

Lol.  The only real way to make sure your data could not ever fall into the hands of others would be to physically smash the platters in the drive.  However writing zeros 1-7 times should make it more than most people would know to do.  Or just reuse it like you said.  

You could just give it to me if selling it is too much trouble    
 
I'd just keep the HD.  They are so cheap now.  You could even buy a new cheap one and put it in so the system is still complete.