budget? would you reuse any other parts from the gateway? do you need a video card?
the cheapest 1366 mobo on newegg is $160 but gives you dual x16 slots for crossfire/sli and triple channel memory. if you're only going to be reusing the cpu then i would go for the 2500k build or get a 2100 to hold you over until ivy bridge arrives.
1366 is dead. LGA2011 has replaced it at this point and the 920 is one of Intel's newly discontinued models. While you might see similiar clock rates between a 920 and 2500k (not really though, 3.8 vs 4.5) the architecture differences between 1366 and 1155 are substantial.
1366 still uses a dedicated northbridge to transfer memory and pci-e data. 1155 does this all on-die which is much more efficient.
1366, while it had a great run even perhaps the best run, is done. It's been done since 1156 and P55.
The only way I could recommend a 1366 right now is if someone wanted 4 gpus on an eyefinity/surround setup and at the same time couldn't afford a 3930K. Not very likely.
the performance gain of a 2500k is nominal for the price over the 920 as the 920 can easily match the 960 performance wise at 3.6, clock the 2500k to 3.6 and there is abou 7 percent difference on single threaded apps but the i7 will still win the odd test... especially on apps that take advantage of multithreading which a lot of benches leave out to encourage you to upgrade... if you have an i7 920 or even an i5 870 or better then you really are just throwing money away for very little performance gain.. its likely to be that way for at least another 18-24 months...