Can I transfer BTC from MtGox to Ripple to buy more XRP from there?
Yes, trivially via Bitstamp.net
And is it better to buy XRP with USD OR with BTC???
Check the Ripple order books to see which has the best prices.
I don't even know how to buy them from within Ripple but there's supposed to be a way.
Seach for one of the half dozen posts here or the dozen or so at ripple.com/forum that detail how to do it.
I can sell you some for: 1 btc/1k xrp
And ignore the people trying to trade XRP outside of Ripple. Almost without fail they're charging 3 to 55 times (in this case) the going rate inside Ripple.