I have an old Pentium 4 laptop, 512M of RAM, that I was net-booting off of a highly modified Diskless MD image which I'm using as a workstation in my workshop. I was using the local disk as swap, to offset the limited RAM. That was working fine off of 10.04, so I think you'll probably be fine net booting it.
I did an in-place upgrade to 12.04, which I'll be documenting over time. I'm sick right now, so I don't feel like doing it at the moment... The diskless image I was using caused me issues in the core upgrade (no real support for Diskless Workstations, only MD's) so I had to delete it. If you're doing a new install of 12.04, things will likely go just as described in the Wiki. Once I got over all the upgrade hurdles, 12.04 on the core has been running pretty well for me (Disclaimer: I'm not running the Diskless MD's yet; production is still on another network using Minimyth).