OK, here's an Amiga-friendly post.
All-in-one new desktop Amigas will indeed soon be available. It remains to be seen how well any particular model performs. Also due soon are upgrades for 'classic' Amigas, to enable long-time owners to run OS4. Perhaps hold off making a decision until they are actually out.
It is still worthwhile buying a classic Amiga, although bear in mind they are pretty old designs now, and so have (for example) slower I/O ports than you will find on current PCs. The same thing will not be the case with a new model.
If you want to take the classic route...
I wouldn't recommend upgrading your A500 as your main Amiga. The 32-bit A1200/3000/4000 models are much more powerful and upgradable. They are all available second-hand for not much money, and so are more cost-effective in the long run. Deciding upon a specific base unit should depend on what you want to use it for.
Expansions are available from many retailers to boost processor power, add extra drives, connect to network/internet, pretty much anything you want.
My own system:
A1200 (in tower case)
Apollo 1260/75 (with 32MB)
20GB IDE HD
2x IDE CD Drives
Mediator PCI Busboard
D-Link PCI Ethernet Card
Various other bits 'n' bobs.
Runs just great. Efficient OS and proper multitasking, you see.
Waiting for OS4 + SharkG3 myself. Promises a very appreciable perfomance boost, and no need to replace mobo, or anything else!
UK retailers I got this stuff from: Power
@Shadowfire: No good Amiga web browsers? Not true!