Since the price on a 2 GB MacBook Pro-compatible SO-DIMM recently entered the atmosphere on NewEgg.com, I went for it. I’m very, very pleased. My MacBook Pro generally performed well with 2 GB, but when I launched Parallels, I’d have to trim down my working set to make sure my system didn’t crawl.
After the upgrade, I can leave my full OS X working set up while Parallels does its thing and it all works fine (allocating 700 MB to a WinXP VM). Once thrashing starts during an OS X / Parallels session, man, game over.