Rather than tell you your rig is a piece of crap and that you need i5 2500k + some high end AMD or Nvidia video card (which was the general theme of the other responses), I'll give you an honest answer. 7300GT is a VERY low end card, and even on a P4 2.8 it's going to be a bottleneck. My girlfriends P4 3.0GHz (lga775, pci express) was generally GPU limited, not CPU limited with a Radeon 4670 which is a lot more powerful than a 7300gt. The resolution you're playing at also makes a big difference (in my girlfriends case it was 1680x1050 as that was her monitor's native resolution)
Too bad your system is PCI express I have a Geforce 6600 AGP card I would have sold you on the cheap which would have been a decent upgrade from a 7300gt. Either way, any modern card for ~$50 will be a huge step up from what you have. The P4 will most likely still be a bottleneck(depending on resolution and how cpu dependent your game is(MMORPG's tend to be fairly cpu dependent)) but it should perform a lot better than what you have. You can probably find a used ATI 4850 for cheap(well under $50) that would be definitely more than powerful enough to max out that p4