EQ seems to run pretty good. I get some weird graphics glitches sometimes with the 8800GT that I never got on my old computer with a 7600GT. Was sorta thinking of getting a 4850 or 4870 video card, but do not wanna spend the $$$ if is is not an upgrade in either performance or eye candy for what I use it for.
The 4850 is a very small upgrade from 8800GT (it's just a bit faster than the 9800GTX+). It's not worth upgrading IMO because they're too close.
The 4870 is a noticeable upgrade, on the other hand. It will offer extra eye candy in future Blizzard games (Diablo 3 , Starcraft 2) because it supports Havok physics. I don't know if WoW cares though, not the current release anyway.
No worries there. The 520HX can handle a HD 4870, especially with a system like that (dual-core, a single HDD). The Sapphire recommendation is "at least 500W". The typical PSU claiming 500W offers 36A or less on the 12V. The 520HX has 40A.