a friend of mine donated me his old laptop, because he knew I was interested in playing with Linux and laptops. The present is an HP nx9030 with following properties: Pentium Mobile 1,6 GHZ (Centrino), 512 MB RAM, no HDD (he kept it because of data), 32 MB of video-memory (shared), WLAN module + some standard ports. Detailed info can be found here. I took a Live-CD with Ubuntu 9.04 and started the laptop. And it simply worked. Nice... I even could use WLAN without any problems. It ran a bit slow, but I could live with it taking into account that Linux hat to do everything (including SWAP) in RAM.
So, my conclusion: if you need an ordinary "working horse" for office tasks (writing/reading mails, typing documents, Internet surfing), than you may take nx9030 without any hesitation. It's designed for Windows XP and works fine under Linux Ubuntu - even without HDD.
I personally have Thinkpad T43p, my wife - T23. But I decided to keep that HP laptop for emergency case. If something goes wrong with ours, we will be able to go online and work even in the current (limited) configuration.
Best wishes from Hamburg,