HP m1050e Upgrade Help needed Please!

Hi All!
I have an HP Photosmart Media Center m1050e:
DVD-ROM
DVD-RW drive
160GB HDD
NVidia FX5500 AGP 8x (Came with it)
AMD Athlon 64 3700+
512MB RAM
Stock Bestec 300w ATX
5.1 Sound Card
(You Can check the General motherboard specs on the HP Site)

I want to Upgrade the graphics card to a 6600GT XFX 128 or Regular 6600 256
( I am not so sure if my power supply will work.. But anybody with personal experience please post!)

I have a 40GB Western Digital 7200RPM hard Drive that I want to use for the windows operations and the 160GB for my media and etc.
(I don't see any Hard Drive Rack because of the Media Drive, but again anybody with personal experience please post)

I want to upgrade the RAM from 256X256
to 256MBX1GB
( I am clear on how to upgrade RAM)

Please help I want to do these upgrades to help play games.

Also has anybody had any experience with the FX5500 Card? How is it with Grand Theft auto or Half Life -- The reason I ask is because I just bought it used and I am waiting on a LCD screen because I've been a long time Laptop user.

Foreget about the cards you mentioned and get a 6800GT or GS or if you're on a budget a 6800XT Extreme.
If I were you Id swap out the power supply for an Antec or Enermax 350-400w. They are about $60.