I did not read the replies (sorry, guys), but I had pretty similar configuration before as you do (AthlonXP 1800+, 512 MB RAM and GF4 Ti4200 128 MB) and I upgraded to 1 GB RAM and Radeon 9600 Pro 256 MB RAM.
1) Turn off antialiasing
2) Upgrade RAM, it's worth it if you're planning on playing more (which you apparently do)
3) Upgrade graphics card only if you want to increase resolution (then nVidia 6600 is the way to go IMO, Radeon 9600 does not cut it for higher than 1024x768). I pretty much regret the upgrade now.
The difference between 1 GB RAM and 512 MB is that when the game is loading, you're experience a LOT of HDD lag with 512 MB and it's good to stop for a while, but there's no slowdown at all with 1 GB and you can play without any problems even when HDD works full-speed (maybe you'll just see some objects later). Typical difference would be starting game in Orgrimmar near AH - you would have to wait like a minute with 512 MB RAM, but with 1 gig you'll just do what you want, while game will be loading in the background.
1) Turn off antialiasing
2) Upgrade RAM, it's worth it if you're planning on playing more (which you apparently do)
3) Upgrade graphics card only if you want to increase resolution (then nVidia 6600 is the way to go IMO, Radeon 9600 does not cut it for higher than 1024x768). I pretty much regret the upgrade now.
The difference between 1 GB RAM and 512 MB is that when the game is loading, you're experience a LOT of HDD lag with 512 MB and it's good to stop for a while, but there's no slowdown at all with 1 GB and you can play without any problems even when HDD works full-speed (maybe you'll just see some objects later). Typical difference would be starting game in Orgrimmar near AH - you would have to wait like a minute with 512 MB RAM, but with 1 gig you'll just do what you want, while game will be loading in the background.