Well, here is my 2 cents worth.
I have a Gas Gas 300 and did have the KLX450 at the same time. I left the 300 in the shed and was swayed over tot he dark side (4 strokes) for about 18 months, until I took the 300 out for an organised trail ride at Kenilworth (the chook chase as it is locally known) and I can let you know this.....
The 300 is the weapon of choice for more spirited trail rides and races. The 450 just beat me up when I got tired. The 300 has more than enough power than you may ever need (but nothing like the 500, so don't kid yourself if you think it is a 500 replacement).
I took the engine out of the KLX450, and shoved the 500 in it, and haven't looked back. For lazy trail rides, you can't beat the KLX500. It took about 12-15kgs off the weight of the KLX, and just hoses the 450's. I couldn't be happier, but be warned, the power of the 500 is the thing that will make you tired, not the weight.
Just my opinion.
TSF