I currently have an asus P5KPL-CM mobo,1920X1080 lg monitor,intel dual core 2180 2ghz processor and a 9400gt ang 2gigs of ddr2 ram.I plan to buy a hd 5850.will it get bottlenecked,and if so,buy how many fps.
The CPU is going to hold back a HD5850 by quite a bit, although the high monitor resolution means the effect will be less pronounced than for a lower resolution.
Either: Take the plunge into the sinister and dark world of overclocking;) or drop a better CPU into the existing MB, it's capable of handling some quite good dual core units and few games benefit from more than a fast dual core anyway:
Some side notes:
That MB is Micro ATX, if it's in a small case you should check to see if the HD5850 will fit.
Overclocking will require a larger cooler, again it may not fit into a smaller enclosure so check before you take that route.
If you are still stuck in the fetid quagmire that is XP you'll not get the DX10/11 benefits of the new card: Neither will work with XP.
If you have Vista you might want to add some more memory as well, especially if you have some or all of its memory hungry features enabled.