What do I need to spend to get a basic small business computer? Any recommendations?

I want to purchase a computer that

a) won’t be obsolete in two years
b) will run MS Office, Firefox, and a few other basic programs without me noticing major problems with speed, memory
c) doesn’t weigh 10 pounds and look like a briefcase, and
d) is relatively reliable.

That’s about it though. I don’t care much about looks, graphics, games, multimedia, snob appeal.

How much will I need to spend, and do you have any recommendations? Will the $500 laptops Best Buy sells do just fine?

Thanks!