Yes, you are likely to experience a major performance improvement (over 10% in most scenarios) by switching to the 64-bit system. Note however that such improvement is not perceived for 32-bit processes.
This is not realized by not using PAE, but instead due to the 64-bit environment itself, which does more work with less CPU cycles. As Hennes suggested in the comments, check out this for details: 32-bit vs. 64-bit systems.
As for the percentage, I benchmarked it myself (though I used Debian, but close enough). Also, a web search returned me this:
Phoronix has taken a look at the performance difference between the
32-bit and 64-bit editions of Ubuntu 13.04. They found that the 64-bit
edition of Ubuntu had superior performance in real-world
As for RAM usage, I also have 8GB and it suffices. My usage scenario is pretty much the same as yours. Of course it depends on the amount of RAM your VM demands.
Looking at your screenshot, and since you don't mention gaming, I recommend you double-check your BIOS to make sure you have no more than 256MB for Dedicated Video Memory. Also from the screenshot, I'd be tempted to suggest you use Firefox exclusively, so this might be worth mentioning since Chrome really uses up lots of RAM.
Now, even though switching architectures will definitely speed things up (which you can test yourself, for instance, with the PeaceKeeper Benchmark - running from a 64-bit Live CD), it is very very likely that the slowdown you have been experiencing is related to the Hard Drive. Defragmentation is not usually needed for Ubuntu, unless you are running low on storage space, so I'd check up on that.