hi! i currently have an i3 540 3.06 ghz and a friend of mine is selling his i7 920 2.66 ghz for a low price! i wana ask is that i7 more powerfull in gaming than my i3 coz its frequency is lower than my i3, and if i do replace it then would my gts 450 be bottlenecked by the low frequency?
The i3 is a fixed speed dual core cpu (3.06ghz) with hyperthreading and dual channel memory support but lacks turbo-boost and is built for socket 1156.
The i7 is a variable speed quad core cpu that features turbo-boost to 2.93Ghz, hyperthreading, and a triple channel memory controller (can move 33% more data to ram than the i3 at the same speed) built for socket 1366
The I3 is a good gaming cpu but the i7 is better for cpu heavy games like BF3.
You will not notice a difference from the hyperthreading or number of memory channels but you will notice tha difference in cpu bound programs & games.
Neither will bottleneck a 550ti
Since they use different sockets they are not interchangeable, the 920 will not physically fit on your socket 1156 motherboard so you would need to get a motherboard and triple channel ram in addition to the processor.
You won't bottleneck GTX 580's in 3 or 4 way SLI with an i7 920. The 920 is a monster still to this day. It's easily overclockable for big performance gains. Took me 5 minutes to get mine from 2.66 to 4.0 for basically a 50% jump in performance. As mentioned above, the problem for you is you'll need a socket 1366 motherboard and triple channel memory.
If you're going to do that, some (myself included) would probably almost push you to buy an i5 2500K with a socket 1155 board for 3 main reasons. 1) You could reuse your dual channel memory on that board. 2) 1155 boards are cheaper than 1366 boards. 3) 2500K > 920 for gaming purposes. I'm not sure what he's selling it for, but you can pick up a brand new 2500K at Microcenter for $180. The 2500K is also a GREAT overclocker. Much better than the 920.
To sum it up, depending on what your buddy is selling the 920 for, I'd bet that upgrading to a 2500K and 1155 board would be better for gaming, use less power, and be cheaper than upgrading to a 920, 1366 board, and triple channel memory.