1. Leopard will run better on your Intel because there's been lots of optimizations made for Intel.
2. I would def, max out your ram because ram is CHEAP, and even if you don't get Leopard, it's still worth doing.
3. Spaces is awesome.
4. Stacks is awesome.
5. TimeMachine is awesome.
6. Leopard's updated Mail.app is awesome.
7. Safari is faster, and thus more awesome.
8. Certain programs (such as CS3 if you use it) are built more for leopard/intel macs. (sorry I don't know how to work in the word awesome here...oh wait just did)
9. Networking is easier than ever in leopard, it's awesome too.
10. Leopard is a wonderful upgrade, 10.5.2 solidifies it a bunch and now would be a good time to get it because IT'S AWESOME.
Do not shy away from Awesomeness.