To provide a little more detail there -- yes it is the CPU that will bottleneck in that. However, I wouldn't bother upgrading in an old chipset, you are looking at replacing mobo, ram, and CPU all at once.
In order to avoid bottlenecking that sweet graphic card you would need to drop around $300 on the above parts (looking at $175 cpu, $40 mem, $90 mobo). (assuming you have a good PSU)
LOL really? $175 for a new CPU? Right now, the core i3-2100 is pretty sweet in terms of performance, surely that won't bottleneck his GTX 550Ti
So, I recommend an overhaul too - core i3-2100 to be more accurate is more than sufficient to handle the GTX 550Ti. decent budget mobos can be had for as low as $50 dollars. Unless he wants 8gb of ram or something like that, RAM doesn't cost $40 dollars - maybe high 20s for good 4GB RAM.
I should acknowledge that technically basch is right here in that you could upgrade for cheaper -- however assuming you are not poor, a major step up to a new system will last you a lot longer and I think is a much better choice.