Optiplex 755 does support Mavericks afaik (and therefore Yosemite should be supported too), but I'm not sure your dual-core Pentium E2160 is actually natively supported by OS X kernel. It really is a low specs, low performance, oldish CPU these days... Check with tools such as CPUID if it embeds SSSE3 technology. If not, no joy...
As you probably know, the integrated desktop GMA 3100 is totally unsupported by Apple's OS but the nVidia Geforce 8400GS is.
There are guides and a bootpack at OSXLatitude, so I suggest you have a look there but, again, you may find that your CPU is not supported, in which case, you could either upgrade to a Core2Duo model or consider Bronya's AMD kernels (they do work).
PS: I guess you meant Q35 (ICH9DO) chipset, not Q32...
Good to speak to you after long time as I was all away from hackintoshing nearly 6months and I'm back now. Thanks for your reply and I've checked with CPUID and can confirm that SSSE3 is supported feature of E2160. And also yes you're right about the chipset which is Q35 Express chipset, not Q32.
Good news for me is I've installed 10.8.2 using myhack and its my first myhack try and I succeded. But I've a small guilty feeling of myhack as I don't know what exactly the myhack does while creating USB installer and somewhere I feel like its similar tool like tmac. I would be really happy if anyone could explain that myhack is not a similar tool compared to any other hackintosh tool that are available on internet. And also I've read that many systems have been successful using myhack and I really appreciate myhack for that. I'm really sorry I'm new to this myhack and today I'm going to go with vanilla install which is my all time favourite with many systems I've installed so far.
By the way, herve, how is your hackintoshing going any new projects upcoming from you.