I was running Ubuntu 7.10 for awhile and it worked quite well for the basic PC-type stuff (web, email, etc).
Then one day I noticed an upgrade to 8.04 was available so I installed it. Well, in my experience everything slowed to a crawl. The problem was 8.04 required quite a bit more memory than 7.10 and that caused excessive hdd swapping.
Maybe they've made some changes since then, or perhaps it was a result of my own configuration, but it's worth keeping in mind that 7.10 may very well be more lightweight than 8.04 on the PS3.