Not always the case. Although it may state a rated power of 850W, its performance may draw less than that. Also you should consider efficiency when buying a power supply. Anything rated 80+ Bronze certified will more likely save you money in the long run than a cheaply made 850W PSU.
Also, MassOwnage has provided an excellent alternative, although you'll have to pay slightly more.
Here is another alternative, but slightly more expensive than the Corsair 650TX,
Antec Truepower 650W £69.99
Specs
24-pin (20+4) main connector
8-pin EPS12V
4-pin ATX12V
8-pin (6+2) PCI-E
6-pin PCI-E
3 x Molex + 1 x Floppy
3 x SATA