Windows XP is using almost all 128 MBs of your RAM just to run the OS.
You need a minimum of 256 more, preferrably 512 more. Your Dimension 4550 only has two memory slots, and can use a total of 1024 MBs RAM. One stick of 512 from Crucial is about $95. A stick of 256 is about $55.
www.crucial.com Your system will be much happier with additional RAM, all your applications will run much better.
I would get the one 512, which will give you a total of 640 MBs.
Your system does have an AGP slot, so no problem there.
I think XtremeJ meant a 9600 Pro, not 9800 Pro. If you can find a 9800 Pro for $95 US, tell me where and I'll buy a dozen!
Your rig with 512 RAM and 9600 Pro or 9600 XT will run AHII fine.
Total upgrade, < $200. A 9800 Pro would be a waste with your CPU and would tax the Dell power supply, a 250 watt unit.