Windows Vista overall score is basically your lowest component score. Without overclocking, 5.5 is correct, you should be scoring 5.9 for your video card and around 5.7-5.9 for memory? Without OCing, that is pretty much the score you will get.
Now, I think it is your "duty" to OC.
There is no reason to get DDR2-800 ram if you aren't going to OC in my opinion. With the stock cooler, you can easily get the E6750 up to 5.7 and maybe to 5.9 in Vista.
In terms of the Vista benchmarks, since there aren't a lot of ranges, it tends to lump things together as a general measurement. It would be in your best interest to use 3dmark or pcmark to get some better reads and then compare with other people's scores to get a better idea.
At 1:1 ratio, the quick and easy OC will get your processor to 3.2 ghz, which is a 5.7 in Vista if I recall. Your temperature should still be under or around 50C full load. Read up on how to OC, make that change and you will see a good boost in performance of Vista.
edit - as a precursor to get you started on OCing, you'll need to set the CPU to Memory Ratio at 1:1. Then make sure the FSB is set at 400mhz. This will get you 8(CPU Multiplier)*400(FSB) = 3.2 ghz, where as before it was 8*333=2.66ghz(stock).