The speculation over whether or not the socket 775 is 'dead' or not has been going on since late 2009 when people were saying that ASRock would be the only manufacturer making 775's within 6 months. Well that's doesn't seem to be the case 18 months later and probably won't be the case for a while longer.
Building a new system around hardware you may already have could be more cost effective for you than buying all new hardware to equip a decent 1156.
I have several computers personally, but one I use the most has a Q6600 in it and has been running non stop going on 5 years. What's so special about a 2.4GHz C2Q? Well nothing really but it is overclocked to 3.2GHz without hessitation on air all that time and thus competing with an entry level i5 CPU for the $150 I paid for it used several years ago. Today you can pick one up on ebay for $100-$120. I don't do any gaming on this system but I run 6 HD's (dual RAID 0 arrays) 1 for storage, 1 for OS, 4 optical drives, 4GB's of RAM, on a Gigabyte P35 MB originally purchased 2006. I get 3064 on passmark 64bit performance bench. My gaming system has a GTX470 in it, a Q9505 @ 3.83GHz, 6GBs of RAM, on an EVGA SLI MB, and it only gets 2200 on the same test.
Many top manufacturers are still making MB's for the 775's now with DDR3 RAM, USB 3.0, and for under $100 (in some cases considerably less) Time will tell if SataIII will make it to the 775 socket.
I've put together many custom systems for my clients in the 10+ years I've been in business, including some i5/i7's but I'm still amazed at the Core2 Quad's ability to OC like crazy, usually by 1GHz or better on air.
Bottom line is unless you're really wanting something special, which would require a 1366 i7 CPU, (as it is the only socket Intel or AMD that supports tripple channel RAM, plus will automatically come with all the latest goodies like SataIII, USB 3, 3x USB power) you can get i5 speeds from a Core2Quad that's nudged a little. As for AMD, their quads and even 6 core CPU's don't hold a candle to Intel's equivalents. I've ran benchmarks on several Phemon Quads that don't even stack up to a really fast Core 2 Duo in processing power/speed.
Also to curb the options of this thread; right now Newegg has 105 different socket 775 based MB's for sale (most of which has DDR3 and are priced under $100) , they have only 39 socket 1366 MB's (over 2/3 of which are priced at $200 and up) and has 110 socket 1156 based MB's (of which only about 1/3 are priced under $100), with the 1155 socket coming in at around 50 entries.