Dude go for the 50D, no doubt the best choice. I have used one myself (and I own a 40D) and it is a solid camera. AF is quick and accurate, the screen is amazing, it may have a slightly slower FPS then the 40D but it's more consistant, i have used multiple copies of both and I still think the 50D seems faster and smoother.
It also handles high ISO's very well, I have seen and expierienced many times that 3200 on the 50 looks like 1600 on my 40D, and if you put on the multiple layers of noise reducing it looks really good.
besides you already have all Canon gear dont you? stick with it!!
I cant say many negative things about the D90 as I have no personal expirience with it, but I do however know a friend that has all 3 (D90, 40D and 50D) and he says he preferes them in this order:
1. 50D
2. 40D
3. D90
I take his word for it as he's an expirienced photographer
__________________
Mike Pics of my rig
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Case:Lian Li with Swiftek apogee watercooling
CPU: Intel Q6600 (2.4 GHz)
Video: NVIDIA GeForce 8800GTX BFG OC Edition. LG 23" 1080HD and Sharp 19"
RAM/Storage:4GB Kingston Hyper X DDR2/320GB(7,200RPM) an 160GB(10,000 RPM) internals, and 320GB External
Sound Card: Creative X-Fi XtremeGamer Fatality
OS: Windows Vista Home Premium (64-bit)
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