Even WoW benefits from a "relatively high end" (upper mid-level, or lower top level) VGA such as the 6800GS ( $200 + / - ).
Your current FX 5200 is almost as useless as an Intel IGP is, but a 9200 is roughly as bad, and an MX is of course, actually as poor as that IGP. If you can't spend $110-120 for a new card, go shopping for something used. Anything from $90 or so, and down, just isn't enough better than the awful FX, to justify taking the access panel off.
So: how much was it you planned to spend?
P.S. The likelihood that a system new enough for a 2.4 GHz P4 missed the jump from 4X to 8X seems remote. You can download AIDA32 from Major Geeks and see if your MB isn't actually 8X, of course, to be certain. You may have been thinking of the last "Might- have- been" AGP incremental improvement that got mostly lost when the MB makers (and VGA makers) rushed into PCI-e.
