I have a old Toshiba Tecra 8000 laptop running Ubuntu. It's got a PII 300mhz CPU and 256MB of Ram and a very slow 6gb HD.
Ubuntu is painfully slow on this system. It takes a while to boot, and even once firefox is eventually loaded, it's slow to navigate pages. Dapper may help in this regard, as it'll have Firefox 1.5 which is quicker, and the next release of Gnome should have much lower memory requirements which may help.
I use a networking card I brought from http://openforeveryone.co.uk/perifs3.php?id=12. This card works well, it's plug and play with Ubuntu, once installed, but Ubuntu is unable to find the card during installation (not a major issue really, network repos will just be disabled by default). Next version (Dapper) of Ubuntu *should* support Centrino via a newer kernel.
Hope this helps.