It's the GPU. The CPU is just fine and gaming doesn't require that much RAM.
I wouldn't touch the 465. If you insist on sticking with nVidia (and you shouldn't), the only card worth it's money is the GTX 460 1 GB, but that's not as powerful as you could get.
Without knowing what resolution you're playing at, we can't really give you a good answer. I know that the best GPU you could afford is the HD 5870. It runs $370, but you don't need to spend that much if you're not playing at 1080p. If your monitor has a resolution under that, I'd get the HD 5770 ($130ish) or the HD 5850 ($255 is the current low price). The 5850 is a bit overkill, but if you upgrade the monitor, it will still be a great performer.
If you have a lower resoltuion monitor, I would buy the 5850, possibly add a Samsung Spinpoint F3 500 GB or 1 TB drive for some added HDD speed for $55-75 more, and start saving up a bit for a 1080p monitor. If you've already got a 1080p monitor, sink it all into the HD 5870, which will be good enough to carry into a future build either by iteslf or with a second one, assuming you build a new machine in a few years.