If given a choice, I'll still go back to XP (in fact, I just did - Reinstalling XP + Linux into a new harddisk.)
Not truely against Vista now though... When it first came out, it was really a nightmare. I've since accepted it. Looking forward to the upcoming Windows 7 (not yet named) estimated end of next year.
If you're going for Vista, DO NOT save on the RAM! Try to go for 2Gb RAM. Otherwise, minimum 1Gb RAM for Vista Home, 2Gb RAM for others. If you're using 1Gb RAM on Vista Premium and above, turn off composite effects (aka. The "Aero" effects).
I agree with you about the ram, but turning off the "Aero" effects is actually counter-productive. The Aero interfaces doesn't use your CPU, but instead your GPU (which is idle for the most part unless you're playing graphic intensive 3-D games or watching HD videos, by which point you're fullscreen anyways so the Aero effect is turned off). The Aero interface is actually a vastly superior method to render graphics, and isn't just a pretty face. It increases stability within the operating system if you have it turned on, so unless you don't have a dedicated graphics chip (Which you should if you're paying for a decent machine, and if you're not make sure you get a machine with one [This means getting like a Nvidia chipset, instead of say something like Intel Integrated Graphics Chip]).
However, you could always wait for Windows 7, coming out in early 2010 but it's already so polished that it could come out late, or even mid 2009. Windows 7 builds upon the Vista framework but everything is more refined, early responses have been extremely positive.
FYI there are no mac laptops for under $600, so even if you wanted one...
Dell and HP have some great personal laptops that are cheap though. Also, if you're using your laptop for just really basic purposes, surfing, email, word processing, etc. You should consider getting a Netbook. They're ultraportable devices that are small and cheap, but are very neat.