I would think that the A8-6500 is comparable to my i5-3550, which has 4 cores running at 3.3 GHz (not overclocked). My last work unit under Win7 64-bit:

World Community Grid	7.32 FightAIDS@Home - Vina FAHV_x2NMZ_A_PRAS_0973734_2284_1	00:32:58 

The longest time I see is 42:19, and the shortest is 27:06, covering a period of the last 5 days.

Yes, I think there is a difference. But if you look at the Linux 64-bit data, it is 0.2 (0.2 - 0.2), so that the min is the same as the max. That means that they don't have very much data. Also, Linux machines are often dedicated, whereas Windows machines are more general purpose and do games, etc. I don't believe the OS difference is as large as has been quoted here.

OK, I have not looked at the CPU difference. Also, the size of the cache will make a difference. I would think that the Intel chips are more optimized for Windows, and the AMD may be more optimized for Linux, since that seems to be their user base.

EDIT: I just ran the BOINC benchmarks on mine, and got:
4/8/2015 2:25:43 PM 4058 floating point MIPS (Whetstone) per CPU
4/8/2015 2:25:43 PM 11589 integer MIPS (Dhrystone) per CPU

The integer speed is probably more relevant, and runs about 23% faster than the 9435.72 million ops/sec for the AMD A8-6500. It seems that the time for crunching the Vina work units is not much different between the two, so the advantage of Linux would look to me to be about 25 to 30%. The problem is doing the comparison between the OS's on the same chip, which is not often done.

Nope, your 64 bit Windows will finish the 32 bit task in 32 bit.

A matter of advise would be to run the computer dry before upgrading.

GL

Well documented on the forums... 4 to 8 % faster on W64 with 64 bit science app.