When configured correctly, Apache 2.0 kicks 1.3's ass.
Especially under high load, switching from traditional prefork to the new worker-model makes a huge difference. If it's just for static content, I would say you can throw as much traffic at it as you want - you'll probably run out of bandwidth before your server breaks a sweat.
In fact, when my galleries get listed at thehun or other high-traffic sites, I can hardly see any increase of load on my servers. Other than bandwidth used that is...
