i've got that setup right now (except the 1.0 and the radeon 7500). 256 will get you by just fine (while doing all of that), but more is deff. better - there are times you can feel you're running low on ram. generally, no. so if you don't have the cash, don't do it. but if you have the cash, you'll be happier.
and if you start using fast user switching, and you have people in your household *cough* *wife* *cough* who continually stay logged in, that extra ram will help out quite a bit, as each open user will be taking up some of that ram.
oh, and if you plan on diddling in garage band, its nice. w/the 256, it just barely gets by...
matt