Well if you're thinking of a thumper a DRZ400 would be a cheaper alternative to a WR400/426/450, and it has electric start (worth the weight!). WR's have (in general) better suspension valving stock than the DRZ, but once you get a drz properly sprung for your weight (very soft as stock) they are superior quality components; forks are best conventional's i've ever used for trails.
Stock to stock, the WR426/450 are more powerful and better suspended than the drz; better suited for a faster and aggressive rider. DRZ is more durable (motor-wise) and they can really rip if you put some time into them.
Wieght also becomes a factor though; although the WR's are technically "lighter" then a drz they don't really feel like it. Drz carries wieght very low, a good thing, especially in the tight trails we all just adore. :)
I am a bit biased though, i guess; I have an 01 drz400e. I really like it and wouldn’t trade it for anything because it is perfect for the type of riding I do; all types of trails.
My 89 KDX is a very fun bike as well, but in the power department it isn’t really even close to pretty much any 400+ cc thumper.
Try searching thumpertalk.com in the drz/wr forums for “wr v.s drz” and you will get more info than you can shake a stick at. ;)