Well I don't think you'll HAVE to upgrade to Longhorn, but you will probably WANT to.
That's really what the Athlon 64 is built for. The performance increase you'll see on today's applications comes from the integrated Front Side Bus (the FSB runs at the same GHz as the processor).
However, if you do choose an Athlon 64, do yourself a favor and wait for the new ones with better power management to come out. If you buy one before the beginning of next year, you'll likely be disappointed with the battery life.
I'm lookin to buy a new laptop and can't decide between Pentium 4 or the Mobile AMD Athlon 64. I'm not a hardcore gamer, so as long as I can play games like Rise of Nations I'm cool. I want the laptop to last for a couple of years. Will one have to upgrade to Longhorn or can they keep XP? Any advie will be helpful thanks.