I could do with a few opinons please. What would you do in this situation. I have the following spec home built tower
AMD Athlon 64 3500+
2gb DDR400 Ram
Nvidia Gerforce 6600GT (AGP)
ASUS A8V Deluxe Mobo
2 x DVD Burners
3 HDD totaling 300+gb
420w PSU

The situation I have is as follows, I also have been given, yes given for free, an HP Proliant ML110 G3 server specs as follows:
Intel Pentium 4 3ghz
1GB RAM
2 x SATA 80GB RAID0
350W PSU

Although it is a server it does have a PCIe x16 slot spare therfore I have the chance to jump on the PCIe band wagon at only the cost of a Better graphics card instead of changing mobo, cpu and so fourth not only that but it's actually a smaller case than my current system anyway.
I plan to use this as my new PC and hand the AMD system down to the boy although I will be taking both burners and I plan to purchase an Nvidia 7900GT I know I would also have to buy a sound card to go with it but what would you do? not only that do you think I will need to upgrade the PSU also?

Any thoughts please?

Yes... a new PSU will be necessary if you get a 7900GT. To be safe... go for this Antec Truepower 550W

Yeah u will probly need a new psu
http://www.extreme.outervision.com/psucalculator.jsp

I would Take a gig of ram from the AMD aswell if i was u
If it supports it that is

If you're only interested in gaming, then putting a 7900GT card in the P4 server would be better than the 6600GT or a 7800GS AGP in the AMD box. But the server has less RAM, a smaller PSU, and a slower CPU than the AMD box. Why not just buy a 7800GS AGP card for the AMD box? It will be cheaper than buying a new card + new PSU + more RAM (Does the server have DDR2?) for the server and be faster in all but the highest resolutions, to boot.

Well if he did this that would leave 1 good computer and one terribal computer(for gaming)
Then he wouldn't have anything decent for his son

That is true. Then if he wants two good computers for gaming, then why not buy a 7800GS for the AMD box and a new PSU, 1GB RAM, and a 7800x or 7900x PCIe card for the P4? That would make two good gaming computers. However the AMD unit is at the end of the line unless a new mobo is sourced for it that has PCIe x16 slots in it (read: an nForce 4 board.)

Yes that would be the best thing to do
That is IF he wants to spend enough money for a 7800gs and 7900gt+PSU+maybe RAM

and that is a big IF

Recall, the ML110G3 x16 is x16 physically only. It is a x8 electrical slot at least per the HP documentation. However, can you run a graphics card at x8 in a x8 electrical (x16 mechanical) slot? Will the server CS shut it down?