Firstly The fx5900 ultra is a fine card and according to tomshardware guide it beats the radeon 9800pro in most benchmarks although to be honest it's fairly even stevens!
What i will say tho is adding more RAM to your system won't really make a diff performance wise as 512 mb is fine for your system. Also buying more pc2700 RAM is false economy as when u eventually do upgrade your mainboard/cpu you'll want pc3200 atleast so you'll have to fork out again!
Whatever card u choose tho will certainly give u a healthy performance boost right now
I've just installed a MSI Geforce FX5900XT after having a Radeon 9600 and the change over was easy.
First thing to do is download the latest Nvidia or radeon drivers (ignore the ones that come with the card as they will be out of date).
Then go into add/remove programs and remove your nvidia display drivers. After a reboot you should be in the default display option (which is pretty ugly!).
Shut down and remove the power supply.
Open your case and undo the grub screw connecting the card to the case and gently rock your card out (before u touch the card, touch the PSU to un-static yourself).
Install the new card and make sure it sat in the AGP slot nice and tight and screw the grub screw back in.
I don't know about the radeon 9800 but if u go for the 5900 ultra the u will need to connect power to the card (for the fan). I had a spare power connector from my PSU and u will too i expect (it's the same shaped one that connects to the hard drive and dvd etc.
Close case back up and reattatch the power and boot her up!
XP will find ur new card and ask u about installing drivers. Ignore all this and go and find where u downloaded the latest driver and open the file.
This will start the driver installation which will cancel out the add hardware wizard which started when u rebooted.
After alot of screen flicking and mucking about it will finally ask for a reboot.
All you need to do now is to adjust screen resolution etc to whatever suits you.