A Used GTX 480, will probably go for maybe $130. If you're lucky.
Brand new they sell for ~$200 right now.
I have a GTX 480 myself, and by no means is it lacking.
I'm waiting until possibly the 8xx Nvidia Series to upgrade, or if the next wave of consoles really push development.
But for now, there really isn't a reason to spend the extra money for an upgrade you don't really need.
And personally, I don't mind dropping down minor graphical settings (just not texture size) in games if it DOES start to lag behind in newer titles.
Keep your GTX 480 I would say, until either the Next generation of card, if they have a good improvement over the current and you really wish to change cards. Or hold out even longer for the AMD 9xxx/NVI 8xx series.
I strongly advise NOT dropping your 480 for this generation of cards, with the next drawing so near. And plus, it's winter, keep the furnace going for now.
See I was waiting to see what the 8xxx series is going to be capable of because it's getting hyped quite a bit (not buying into the hype until I see some real world test figures though).
I'm quite happy to drop circa £300 (if I can get £80 - £100 for the 480), but I want to be 100% sure that whatever I buy is going to last me as long as the 480 has - which by the time the 8xxx series is out is going to have been about 2.5 years or so if memory serves.
I have been leaning towards the 670 because so far I've not heard a bad word about it, but I don't want to spend the extra £70 - £90 if I don't have to.