I bought a pc from gateway (

) almost 2 years ago and would like to upgrade the video card and memory without starting from scratch. I do have a nice widescreen 19" lcd monitor that runs 1440 x 900 resolution but framerates hover around 20 fps at that setting. I'm getting 60+ at 1280 x 1024. Textures are set at 512.
Here are the system specs:
windows xp
Core 2 Duo E6400 (2.13 ghz)
SDRAM 1024 ( 2 x 512 ) DDR2/667
intel motherboard P965
250 gb 7200 rpm SATA II 8MB
400 watt power supply
NVIDIA GeForce 7300 video card
onboard sound
The only pc game i play is AH. I want to run on 1024 textures at 1440 x 900 resolution AND get good framerates (60+). I will swap out the power supply or add a sound card as necessary. How much memory would you recommend? I hear good things about the 8800GT. Can my system support this? Is it overkill?
If you are kind enough to reply, please do so in very plain language as i consider computers to be black magic (all those specs typed above - I don't know what they mean

).