Well, to be honest I myself was asking myself that same question aswell, should I upgrade just the GPU or buy a whole new rig?
When I looked at my old time rig, the MB was outdated the Processor was outdated, so I myself am going to buy a full fletched machine from the get-go, my friend however decided to go for the 8800GTS and use that as a pit-stop to save up for a massive overhaul.
It’s actually his teaser for “things-to-come”.
So if you want to wait for a new system, you could just go off and buy the 8800GTS or you can save a few months more buy a new PCIe 2.0 MB and go with a 4850, or 4870.
It’ll cost a little bit more, but if you ever decide to buy a new card you can just go and buy any card you wish, for now the 8800GTS seems to be the highest-end card that you MB will fully support.
No, you do not need to upgrade your motherboard to fully utilise any of the current generation single GPU cards.
A 16x PCIe 1.1 slot still has plenty of bandwith and will in no way hinder an 8800GTS 512.
7900GT => 8800GTS 512 will be a HUGE jump in performance!
You should definatly upgrade but consider getting a 4850 for about the same cost.
It will give you about the same or a little more performance then a 9800GTX+ for quite a bit less.