Performance issues with my xfx ati radeon HD 4890

i recently upgraded from a Nvidia 7600GT 256mb gfx card to an xfx ati radeon HD 4890 1gb card. i play WoW and was seeing low fps in raids and cities/ highly populated areas. For Example: Dalaran/Raids: i was getting roughly 15fps and lower. That is unacceptable to me seeing as this fps issue really affects my play. so after upgrading, uninstalling the old gfx drivers, installing the new drivers for the ati card, i started up WoW. Immediately i noticed a slight graphical enhancement, but strangely no performance boost. i have been told that since WoW is roughly an older game in terms of graphics, i should be able to max out all the video settings and see no decrease in performance. However, after maxing those settings, i did see a decrease in performance. Strangely enough as well, i did not a see a rise in fps; still hovering around 15-19fps and that is in cities/raids. if anyone can shed some light on my issue it would immensely appreciated.

Motherboard: Abit AN52 nForce 520 AM2( this motherboard does have a PCI express x16 slot which is where the video card is in currently)

Powersource: 500 watts

Processor: AMD Athlon 64 X2 Dual-Core 4600+ 2.41ghz

RAM: 4gb

Operating System: Windows XP 32bit

Your PSU, what brand? model?
DId u connect both of the power connector for your card?

both power connectors are connected and the brand/model of the PSU is an Antec Earthwatts EA-500D

and what about if u play without v-sync is on?

many people get trouble when playing WoW...
what about the other games? can u max out?

i didn't know more if every things that i told u aren't help at all...
Anyone?

you probly should upgrade your cpu