Selling the GTX a month ago would have been worth it.
You have sadly now missed the boat.
Keep the GTX and pick another one up cheap when the GTS hits.
If I could get a a 8800 GTX for 170 ish then I would sell my GT tomo.
Unfortuately the GTS will be the card to get making the GTX redundant very soon.
Hardly, the 8800GTX was around £300-350 even before the 8800GT and still is, as for the 8800GTS, it only has a 256bit bus so the 8800GTX/Ultra still have an advantage here and that’s enough to differentiate them and help them maintain a high price tag, plus it has 768MB of memory which is also an advantage over the 512MB version of 8800GTS, I see no reason why the 8800GTX/Ultra will lose their price premium but yes they will probably drop slightly but not much.