Newegg.ca - Computer Cases,MicroATX Desktop,matx - these are pretty expensive but they are really small, yet big enough to provide some decent air-flow and prevent overheating.
Newegg.ca - Computer Cases,MicroATX Mini Tower,matx - these are priced better and still not too small. I wouldn't consider them too big either.
Newegg.ca - Computer Cases,MicroATX Mid Tower,matx - These are dirt cheap, but a little bigger and I personally don't trust PSUs that come with cheap cases... I read a review where they looked at the cheap 400-500w sub $40 PSUs and most of them blew up and failed miserably. 1/2 of them couldn't even get over 250w when rated over 400w.
They guy who was going to sell me a used case flaked out on me and I haven't found one since, so this is what I'm buying for my cheap build:
Newegg.ca - APEX TX-381-C Black Steel Micro ATX Tower Computer Case
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Comparing AMD vs Intel is a little tricky as real cores > fake cores, but Intel performance > AMD performance. Depending who you talk to, some people will assure you that 3 years from now we'll still be running most daily tasks with 1 or 2 cores. I'd rather be on the safe side and assume that in 2 years, 90% of our programs will be multi-threaded and capable of more than just 2 cores.
I don't know if any of the CPUs you'll look at don't have virtualiztion, but just since you're in an office setting I'd point that out as you may need it. As for trying to say which is better? Some programs just like intel better than AMD and multi-threaded programs will prefer the real cores on AMD procs compared to hyperthreading.
I'm really no expert in this area, so I can't give you a straight answer! If you do go with AMD, I'd spend the extra $ to see if you can get and AM3+ socket that will work with bulldozer in the future. Also consider the Phenom II chips as they do have L3 cache (if you need it).
It looks like the Phenom II X4 925 ($125) is a solid match against the i3-530 ($115) and beats it at any true multi-threaded programs, just for $10 more. At $100 the Athlon II X4 635 is about on par with the 925, but without the L3 cache. If you could find an X4 630 for at least $10 less, I'd recommend that over the 635, but not a big deal. As I said before, the great thing with going with AMD is how many CPUs are compatible with a single motherboard :).
My PSU is on back-order :( so I can't bench my build today, or else I'd tell you how that $60 gigabyte mobo performs ;)