Unless you already have that processor, you will be hard pressed to get a recommendation for building a LGA775 platform machine. Your money will be better spent on an AM3 or LGA1155 platform. ...or possibly buying a used motherboard and memory.
That being said, the links below are for the older LGA775 and DDR2 memory. There are LOTS of options out there...
It's probably more expensive to build a new LGA 775 machine than AM3, LGA 1156, or LGA 1155 because LGA 775 parts are barely in production and growing scarce. Depending on your budget, there are some great bundles out there for some parts. If you post your budget and your needs (gaming, video editing, home theater PC, etc.), we can recommend an optimized build.
Any advise on operating system? Windows7 32 vs 64?
Is that a sockeye salmon?
Well if you want to Play Crysis, Go windows XP 64 bit or Vista 64bit.
I heard Windows 7 has issues running Crysis..but I believe patches are available to
fiz that(don't know for sure,just going by what i've read and seen).If you want to stick with what you've got
,get a 775 motherboard that IS ABLE to run DDR3 memory,max out your memory.To play Crysis with no lag,grab the E8400/Q8400(both are now available for around $143.00)(June,2011) Or,if your rich and looking for one the best in top performance in the 775 platform get the Q9650($334.00).I'd personally go for the Quad core/Q8400($143.00)..since Crysis was first built to run on a Quad format, for best performance.Then,get a SATA II/III 6-700 Gig HD(Samsung F3/F4)(if you can find one)Considered the second best HD to the Velociraptor, and in some instances even won over the Velociraptor.Next,grab a high speed 24x Double Layer CD/ROM or DVD/ROM drive or a combination of the two.(Sony Optiarc/Asus/Samsung-the best on the market),once again,for Memory,get DDR3 sticks with the lowest CL(cass latency)specs as possible for super fast ROM/MEM recovery.Grab the best in heat suppression/dispersal for your CPU,the new Corsair H-80/H-100(I believe their out already)Personally id buy the one with the dual fans(H-100/$175.00).Make sure you have fans all over the place to get rid of the heat from inside your gaming tank.(front/Rear/Top and on both sides if possible this will extend the life of your motherboard especially if you decide to over clock it a bit to say...3.4 Ghz.Finally get videos cards with/Cuda/PsyX technology why?The technology,releaves pressure from your CPU,so that it doesn't have to do all the rendering calculations on it's own,the video card now does all of that.More likely you'll have to go SLI for that.But if you do,you will not be dissapointed.Smoke,Artificial intelligence,Lighting,Shadows,Tessalations, are all tweeked to offer the gamer the very best in graphics capabilities for a more realistic and a more emmersive experience...no one paid me to say this..but research it for yourself and you'll agree as well...other than that make sure you have a power supply capable of handeling the loads you put on the motherboard, if you go with SLI,(adding two V.C's) Get the best,reliable and "LOAD TESTED PSU" with the best rating available,so as not to damage your motherboard/CPU or that may even cause blue screens and other issues with your PC components!!Get AURUM power supplys(my opinion/"load tested" and approved to 80 plus gold rating!) 800 Watts which should be more than suffice...good luck!!Happy gaming!!