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When I looked at my old time rig, the MB was outdated the Processor was outdated, so I myself am going to buy a full fletched machine from the get-go, my friend however decided to go for the 8800GTS and use that as a pit-stop to save up for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.