If you're looking for a long-lasting system which you can easily upgrade along the way then I would be tempted to go for an i5, maybe the 750 would be a good choice. The LGA775 socket isn't going to have any further processors built for it and especially since ur doing a new build I would go for the i5 just for the added future-proofing. The i5 750 performs very well in games and will breeze converting movies, photoshopping and anything else you can throw at it.
On the other hand, if you're budget is a little tighter then why not look at an AMD Athlon II or Phenom II X4 chip. They both offer very good peformance for the money and I think the AM3 socket has a little more life in it than the LGA775.
If it were me I would be heading the AMD route but of course it up to you
Good luck whichever you choose!