If it came stock with 667MHz you might want to research what RAM speed the board can utilize. For that matter you might want to make sure your board can take 4 gigs.

If you pop four gigs of RAM in there and the board runs at 667 max, the higher speed memory will "dumb down" to run at 667MHz anyway. However, it can help quite a bit.

You may also consider upgrading to the 64 bit version when 9.10 comes out next month. Also consider creating a large swap partition. The Ubuntu docs recommend having a swap partition of greater size than you have memory. I personally disagree, but if you have the space to spare you can try giving yourself a 4-6 gig swap partition.

Ubuntu is surprisingly memory hungry, so I believe you'll benefit from the RAM.