Before Northrend you'll be fine. In northrend you'll run into memory issues.

Outlands may have some memory issues, not as many as northrend.

If you upgrade to a 64 bit OS you'll likely immediately see 4gb of ram instead of 3.4 (since I can't think of a config to get to 3.4) which would help everywhere. You'll likely run into problems getting Linux to multibox with most multicasters, though i've heard of it being done.

One thing I find amusing is the specs you listed used to be the zomgtopend multiboxing specs...