it's likely that your 6gb ram kit is triple-channel, but your budget is almost assuredly going to put you in a dual-channel configuration (i5 or AMD). if you can return that ram for a good 4gb kit i'd recommend it.
are you planning on SLIing your 260 with a new 460? or do you want a sli board to get a second 460 in the future?
Im wanting to just buy 2 460's, and I just checked cpuz and it says i have dual channel right now.
weird. maybe 2x2gb and 2x1gb sticks?
anyways, check and see if the ram is ddr2 or ddr3. if it's ddr2, you'll need to get new ram if you upgrade to an i5.
there's a 2x Galaxy 460 package for $350AR on the egg, and you should be able to get an i5-760 from microcenter for $170. that leaves you with $180 for a mobo which should be enough (assuming you don't need to replace your power supply). i'd recommend a 650w PSU or better.
edit: if you're only buying one 460 right now you obviously have a lot more $$$ headroom