like will i be able to play games 6months from now?
I guarantee that card will run any games in the next 6 months. That's not an issue. The question is: do you have a problem running games at less than HIGH settings? If the answer is no, a decent agp card - the x850xt being the fastest available - will do you fine. However, it might be good idea to look at slightly slower agp cards if you're concerned about the price/performance ratio.
If the answer is yes, then pci-e is obviously the only way to go.
$400 for a mb/gpu is about right ($100 mb $300 7800gt), but remember that you'll have to make sure the cpu you have is compatible which means considering the age of the cpu and whether it will bottleneck a top of the line pci-e video card.
I don't know much about Intel cpus, except that they suck for gaming. If you're gonna get a new mb anyway, maybe consider an extra $150 and go amd. Unless you have intel specific ram
Again, I recommend doing some research on real world performance gains of an x850xt vs, say, a $100 cheaper x800xl or upcoming 6800gs (yes, agp).