The only thing you can upgrade to with $250 is a GTX 760 because GPUs are expensive these days, so that's pretty much your only option. A new CPU is going to require a new motherboard, and all other GPUs are priced at like $380 and up -- Or even a 3770K is $330. If I had to guess though, I would say that the GTX 460 is probably the weakest link, which will then be followed by your CPU w/o hyper-threading once it's replaced.

Also, don't overclock your CPU with the stock Intel cooler because it's only going to heat up very, very quickly.