best graphics card?

We are building a PC for our son for X-mas. His setup will be: Intel D865GBF, Intel P4 3.2, 1GB RAM, 160GB Western Digital, with XPHome SP2. We need to know which is the best graphics card to install on this system for an avid gamer who plays online RPG's like Runescape, Warcraft, & Diablo? We are looking to spend up to $300. Any good suggestions for us? We undestand that MB will not support PCI-E. Which other would be best?
Thanks in advance!
Shannon & Chuck

 

The best AGP card would probably be a 6800GT:
http://www.newegg.com/Product/Product.asp?Item=N82E16814122195

But, if you wanted to save a little money, you could go with a 6600GT and get good performance as well:
http://www.newegg.com/Product/Product.asp?Item=N82E16814121195

 

for 300$? check out newegg.com and look for 7800GT, i think theres one at 319$ which is an excellent buy.

 

Get a nvidia based geforce fx 5700 or higher video card.

 

The 6600GT will outperfom this in everything. 6800GT will outperform the 6600GT

 

...and the 7800GT will outperform the 5700/6600/6800 in everything

 

Does name-brand make a big difference? eVGA GeForce, ASUS GeForce, Leadtek...etc? Also, how important is 128MB or 256MB? NewEgg does not have GeForce 6800GT in stock. Cannot seem to find GeForce 7800GT in AGP, only in PCI-Express. Is there anyway to allow our MB to accept PCI-Express, as the best ones seem to be that? Thanks for all of your suggestions so far! Just haven't been able to locate a place to buy one yet online . Any more suggestions? Thanks so much!

 

I miss read OP, though you said "I understand mobo will support PCI-E. I guess 6800 Ultra / GT is the only way to go for you. There is a diffrence between 128 and 256 only when you have high end cores, IE 6800,7800 etc...