Not sure where the 50D has a poor reputation nor why people still recommend the 40D instead. Perhaps this is because of the default settings you need to change. DP review got caught out with this, but still seems to be the often quoted authority.
CT? wrote up the settings to change on here. Perhaps someone would provide a link (I've forgotten them).
I went from a 300D to a 40D and then to a 50D when the 40D shutter failed under warranty after 18 months and 30K shots.
The 40D to 50D is a great step up. 15Mp means for sport/birding etc you get more chance to crop to get the shot (especially for birds - blooming small things).
The screen is higher res, meaning I've more of an idea if a shot is in focus or not.
I thought the menu on the joystick was a gimmick, but it's actually really useful.
The digic 4 processor, write speed to flash, high ISO/low noise are again brilliant reasons to upgrade.
Microfocus adjustment is brilliant. I've only had to tweak my 100-400 settings but it's given me more keepers.
The only downside is you now have 20-22mb raw files rather than 10-12mb, so you eat up your disk space quicker.
Seeing as it's a reasonable small difference compared to lens choices, I'd recommend the 50D everytime.
Lenses
Nifty 50 works great on the 40D/50D. I've had some great portrait/product shots using this lens.
70-300 IS will be fine for starters, nice lens, but the power of the dark side L lens will be calling