Gulftown supports instruction sets through SSE 4.1 with AES encryptions instructions if that is important. 135 watts consumed from your power supply is fewer watts for add-in boards.
Sandybridge 2600k You can play with overclocking if that seems interesting. Sanybridge might have a dual socket solution but you probably need the 2011 socket with the extra pins and cost.
95 watts consumed from your power supply leaves 40 extra watts. You can either use that for more graphics with an addin board (gaming) or for a slightly lower utility bill and cool running (quieter fan).
If you are careful about the 1155 socket and motherboard design choice, Ivybridge might be a direct plug in for your current board. You can be the first on your block with the new system.
Sandybridge supports the same Gulftown instructions plus the new 256-bit AVX vector instructions. There is not much that exploits the AVX yet but there seems to be an industry wide migration to exploit new instructions.
They are similar in base frequency, cache sizes and bus bandwidth. I think there was some improvements in the internal cache design on the chip that might be visible to benchmarks.
Hyperthreads give you about 50% to 80% of the compute power depending the application mix.