looking for different bike - upgrading

Here's what you should look for.

Four-stroke:
Honda CRF250X
Yamaha WR250F
KTM 250XCFW (formerly the 250EXC RFS, but the 07 and newer models are way better)

Two-stroke (my preference):
KTM200XCW (formerly the 200EXC)
GasGas EC200

Don't be afraid of GasGas - parts are cheap and easy to get and the bikes are very, very durable (my 200 is amazing for the number of hours on it).

Dont rule out 250cc ..or even 300 cc two strokes. My 250 has such nice smooooth power, yet can pull strong when i want it to - I just love it. My son's KTM200, while set up nicely, does require more attention to keeping it in the power band. His bike is a MXC, so has a close ratio gear box - which i think makes it easier - it does kill top end speed - but who cares about top end speed unless you are DS riding all the time! A 300 is a torque monster - you can totally be lazy, and run two gears to high all day long, just twist the happy handle when you want power. (Kzoo mentioned not wanting to shift so much). A 300 however can be more tiring to ride as it seems to always be ready to yank your arms off.

Oh FWIW - I was just yankin Silver's chain about the Gas Gas...great bikes from what I have seen of his - and from what he has told me, outstanding dealer support and parts availability.