As a (recently former) WoW player, I wouldn't wish a machine with only 512 MB of RAM on anyone. It was so-so with WoW in 2005, but as more patches came out, it was worse and worse. The single best upgrade I ever made for improving WoW play was to go to 1 GB of RAM. Trust me; use no less. I'd recommend 2GB if you can afford it.
The second best upgrade was upgrading the video card. I originally had a 9800 Pro with 128 MB and it was fine. It seemed, however, that new patches made a better graphics card necessary. I'm not sure the graphics themselves got that much better, but more items were present to be rendered at the same graphics settings as before. At least that's my guess.
Eventually, the 9800 pro was really dragging - especially in the 40-man raids. I upgraded to a 6600 GT with 128 MB of RAM. That helped a lot, but again, as patch Tuesday after patch Tuesday went by, it seemed to be slowing down. I know something was changing in the graphics area, because after several WoW patches, I needed to upgrade my graphics drivers to resolve crash issues. I was able to play in 1280x1024 mode with the 6600GT (with most of the graphics turned down). (WoW is still pretty decent looking at that level.)
As the majority of ATI X1300s have significantly lower benchmarks than the Nvidia 6600 GTs, I'd recommend an ATI in the X1600-X1650 class or better or an Nvidia 7600 GS or similar. Tom's Hardware Interactive VGA Charts
can help you explore the differences.
I played WoW on an Athlon 64 3200+, so your core processor should be fine. 80 GB is small for a hard drive, but if you only
play WoW, that should be plenty.