if my "off board" buddies stumble across this, they are going to say Plowboy and his broken record...
Let me tell you my experience, and you decide what you would do...
I have a 2.4ghz p4, on an OLD motherboard, that when I 1st bought this system, it I had the geforce2 mx 32 meg card, because I already had it. (BTW, before they made the distinction into the mx200 mx400 etc.)
Ok this was good enough for the Sim racing games I mostly played for a year or so.. but, I keep watching prices fall on the hot cards as newer hot cards come out. So I move not long after the NVidia 6000 series cards came out, to the ti4600 128 card for 1/3 the price mind you. I saw awesome performance gain BTW.
Couple years go by, I still think i have room to upgrade and bought the 6600gt. I saw no difference in performance, but I could see more tiny details because of the Direct X versions the cards were written to do. Well I'm disappointed and find out yep, pretty much bought a card not better than the ti4600. I need more gaming HP, and I need it relatively cheap... so I thought, Ill try one more time because my new favorite SIM was not performing very well {mind you I had 1gig ram, fast as I could find hard drives you know, this thing wasn't cutting edge, but I thought it should been decent game wise...}
Anywho, so I bought me the 7800gs(oc) with 256 meg (might be the 512 version, but my mind is now challenged on that fact) still gained NOTHING. I was almost sick tossing money down the rat hole, I sure as heck wasn't glad I spent that money (250 and change and it was not bleeding edge like the 400+ dollar PCI cards out now. So obviously, my motherboard and CPU holding me back anyhow.
SO what do I do, I already spent a pretty big wad, somewhat foolishly, on the AGP 7800 gs/oc. Well I found an Asrock dual VSTA (has AGP & pcie slots) got me a e6400 core duo (225 chip & 75 motherboard) and got a HUGE gain, and get to use my AGP stuff until I save up again. I tripled the gaming performance plus load times for just friggin anything.
What I am saying (contrary to many) is if you already have a decent HIGH end AGP card, and don't want to just throw it away, to jump to PCIe, I suggest then, look into that motherboard with both AGP & PCIe slot, and for less than the high end GFX cards cost right now, gain on helluva lot of performance, for 300 or less. but if you don't have at least.
I'm having trouble thinking your 2.8 is that much "faster" then my 2.4, but it could be, but on one of those VGA charts, if you don't have a AGP card, that is equal or better than the 6600GT with 256 meg ram, just start saving up for all new with PCIe, IMHO.
good luck