First off, Shader model 3 will be very heavily used in next gen games. Sure, maybe the UE3 demo ran slowly, but you have to consider this:
1. It wasnt completed yet (and still isn't)
2. that was on like uber graphics settings
3. Whoever makes the UE3 engine said "a 6600gt will run games based off of the engine fine"
Now, that could mean Low settings on 800x600, but thats better than nothing at all. A 6800gt would probably do a good bit better.
Also, FEAR has a very nice performance boost on SM3 (6800gt beats the second best card out, the x850) and its also funny that the game is an ATI game. Last time i checked, ATI games ran better with ATI cards...and with SM3, it doesnt.
if that doesnt say something to you, you're weird.
Also, the reason they used 6800gts in SLI to make the UE3 engine was for two major reasons:
1. Together they are powerful. Very powerful.
2. Shader model 3. Duh, how else do you get the great graphics.
And yes, Shader model 3 DOES have a nice IQ boost over Sm2. Its due to features like Displacement mapping (something that i'm excited about, and you should read up on if you dont know about it) and extremely long shader code lengths (65000 vs sm2's 600). That allows for some really nice looking shaders and doesnt kill performance because the shader codes dont have to be split up like in SM2 (because they can't fit sometimes).
So basically, yes, you NEED a Sm3 card since you said "...but I'm thinking about buying a card to last me for a few years."
Also, if you get a 6800gt, it does GREAT for now games. No need in getting the x800 whatever because the performance numbers are well above 60fps in most games. Plus, if people like munky are wrong about Sm3, then Sm3 will benefit the 6 series cards very well in next gen. If we're wrong about Sm3, then Sm2 is no better off.
So at least you're safer with SM3.
I don't even recommend reading up on SM3 because you plan on keeping the card for a while. In fact, one website says "there's no sense in buying an ATI card at the moment." and that was from a good source.
Even if the ATI cards are nicely priced and do well in today's games, they are missing a key feature, SM3. Just to let you know, Microsoft is the one that set the standard for SM3. So, next gen, Sm3 will be standard.
In fact, the xbox 360 only runs on SM3. And its a console, but consoles are very much like PCs now.
So i hope you take the smart advice and get a 6800gt or something.