I am looking to build another new computer for my nephew, he is going to be doing some gamming and some every day internet things and a few programs. here is what I came up with so far. I prefer to stay with AMD since that is what I have always used

Is there anything I could change and still stay within the price of around $800. I am in the US

COOLER MASTER Centurion 5 CAC-T05-WW= $59.99

GIGABYTE GA-MA790X-UD4P AM3/AM2+/AM2 AMD 790X = $219.98 cpu/mobo combo

AMD Phenom 9950 Agena 2.6GHz

OCZ GameXStream OCZ600GXSSLI 600W ATX12V = $80.00

SAMSUNG Black 22X DVD+R 8X DVD+RW 16X DVD+R = $25.99

HIS Hightech H485QT512P Radeon HD 4850 512MB = $145.00

SAMSUNG HD642JJ 640GB 7200 RPM 16MB = $59.99

OCZ 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 = $46.99

Total $744

My mother has an older HP computer that she is wanting to play some games from big fish on, her computer lags when going from one level to the next to the point the graphics wont load,, I am not sure about it but I think the onboard video is not good enough, the computer has a intel celeron 2.93GHz process or and a msi motherboard with 512mb of ram,

Can someone please help me and recomend a good agp video card for under $50 that would work for her.. or if you have other suggestion as to why her computer lags so much even though it triples the system requirments for the games she plays...Thank you