I agree that 250 four-stroke is the way to go for your first trail bike:
Yamaha WR250
KTM 250 RFS stoppped making them around 2006)
Yamaha TTR 250 (with mods a great trail bike)
Honda CRF250X
New Husqvarna TE250 looks brilliant but is pricey
New KTM 250 exc-f brilliant but more of a race bike than trail bike
Quite a few manufacturers are producing bikes that offer the perfect balance of power and weight for trail riding:
KTM 400 exc-r
Husaberg 390
Husqvarna 310
With any of these you'll never be short of power but they won't be as tiring to ride as a 450. It is worth noting that today's 400's are more powerful than 450's from 3 or 4 years ago!!!
Whatever you go for make sure that you have the electric start button or you'll regret it!