I would not spend money on a 4000 series due to the 800MHz FSB. On the other hand, the C2Ds do allow the use of cheaper 533MHz ram with a much smaller performance loss than the AM2 CPUs. If the OP were interested In OCing, (which he does not appear to be)) either the $67(US) X2 3800 or $119 E4300 would serve his fairly well, however the 3800 (while it would not reach the same levels as the 4300) would save him $50 to invest elsewhere.
There is no point in getting the 650i Sli chipset, as far as your $200 price budget you going way over that with talking about getting a 650i, E4300 and $50 heatsink.
As far as the future goes no one really knows what it looks like, right now i can tell you Intel is holding a steady performance increase while AMD is stagnet right now their new architecture could eclipse the performance of Core 2 Duo and all AM2 mobos would be compatible.
If you want to go the Intel route then a Gigabyte P965 DS3 or S3 will provide you with a solid board and good overclocking abilities.