If you are experienced with things like building computers/replacing motherboards then getting one that can OC is a worthwhile idea if you don't plan on upgrading for a significant amount of time but that processor at 3ghz is still passable. There will definitely be a CPU bottleneck until you upgrade the monitor though as lower resolutions won't push these cards at all really so the CPU will end up being the limiting factor. The frame rates should still be high though so it won't matter much.
In any case both the GTX 460 and the HD6850 are good cards for the money and will do well at 1080p and very well when overclocked.
I would go ahead with the graphics card upgrade and not get a new motherboard. Save that money for a complete system upgrade to an i5/i7 or even Sandy Bridge. You can still use the same graphics card in the new computer.