If you don't mind playing future games at lower resolutions and settings as the more graphically advanced games that are going to be released (within the two years you plan on keeping this card) will most likely render the 6600GT rather slow.
Your choices:
1. Get an X800XL - a really fast card roughly equivalent to a 6800GT and at a cheaper price (~$300). Lacks SM3 but that is arguablly a nonissue right now. But the future games will need it for the most eye candy.
2. Get a 6800GT - about the same performance as an X800XL and does command a higher price ($370-$420 or so). However, most can be overclocked to Ultra levels, have SM3, and can be SLIed. Thus even if the single 6800GT is too slow for future SM3 games, you could purchase a second for cheap and it will most likely run just fine.
3. Wait for the R520 series and pray that they do not paper launch them (as in announce in the summer and release in the winter or some derivative thereof).
4. Get a 6600GT - Will be slower in future games and limits you in some current games. I wouldn't go this route.
Since you are (I assume) saving money up until June, I would wait until then to determine what the graphics card market looks like and then make a decision.
