as for 64/32 bits, I think your graphic card is NVidia + recent enough to provide gpu acceleration on flash video (well, only in youtube actually ), but this only works (as of today) on the 32 bits version..

On the choice of distribution, well I didn't test many of them so I can't really tell, but I'm using Ubuntu my desktop, home server/htpc and netbook, and it works quite smoothly. All with unity (I've tested kubuntu a bit but it was laggy on my desktop, so I came back to vanilla ubuntu)