The more expensive ones will have father ranges, things like that.
They're all made in the same places, with the same few chipsets.
Wireless sounds great, but due to losses it's not as fast as ethernet. It can also be more intermittent, with interference from cordless DECT phones sometimes causing issues.
The best solution all round is to put the router next to your computer, with an ethernet cable between them (a few pounds), as rojash says.
If you really need to use wireless (can't have the router next to computer for some reason), then the PCI ones are better, if you're ok with taking the lid off your computer. USB will do though, just not quite as good.
Look for ones that give 'g' speeds.. .11g is the technical specification. They'll all do this though.
54Mbps is plenty. Your Tiscali speed is "up to 8Mbps" or something like that -- and 54 is quite a lot faster than 8! So the bottleneck, so to say (where it'll all slow down) is your internet connection to Tiscali, not the wireless.