Best Laptop GPUs right now (please note some of these are only just coming out)
480m>470m/5870m>460m
Depedant on title and settings, theres usually not a lot between the 5870m and the 480m however, and due to the massive cost differential, you can sometimes get a crossfire 5870m machine, and likely soon the 460m SLI in a similar price bracket to a single 480m powered machine. Thats economies of scale, and niche parts for you!
There are also dual card setups available in DTRs, which can actually be quite potent and up there with the gaming desktops, but they cost...well...a lot, and a desktop will always be faster.
As a rough guideline for performance, the 480m and 5870m are about in the region of a desktop 5770, so you'll be able to play anything out there on the market, just dont expect maximum settings, 1920x1080 AND AA.
The dual card setups are around the same region as a desktop 58xx series card in games scaling is decent, so surprisingly potent for something you can unplug and walk off with (and in most titles actually capable of 1080p, max settings and often AA), but again as I mentioned before you pay a premium for this, a real premium, and as DTRs, they're not quite what you're classify as truly laptops, although certainly portable.
If you have the cash, the MSI with the 5870m OcUK have in stock is one of the best relative bang for buck machines in the UK, you're getting a lot for a grand, compared to the rivals. (Its more expensive to buy gaming laptops in Europe compared to the US).