Apparently the Ion-based netbooks are capable of playing those games. It all depends on how portable you want it to be.
I myself am in a similar predicament, I use my 360 for all the main gaming, and want a portable lappy for taking to uni, for the lib etc. Considered an Acer 1810 but it has terrible graphics power. Ion netbooks on the other hand have good graphics capabilities but are choked by the Atom processor.
Depends on your priorities. Light gaming plus excellent battery life and portability=netbook with Ion. If you already have a capable desktop I would recommend the netbook. Getting the HP Mini 311 myself, has great reviews. Check out the "sub £250 netbook" thread