A few quick responses for you ...
1 - You can really upgrade to 20s or so - if you can stand the ride. I actually got rid of the 19s on my ride because I found they were a bit to harsh (though I have a sport suspension as well). I happen to like M68 style rims, which are 17s and I use them the most right now...
2 - I have 1 set of OEM m68 Rims and 1 set of REPLICA m68 Rims. Visibly, they look almost identical, except that the OEM rims are staggered (rear rim is wider then the front), and the REPLICAs are the same size all the way around. I use the REPLICAs as winter rims - cost me about $600USD. You can probably find them a fair bit cheaper on eBay.
3 - Didn't check to see were you live, but I used "The Wheel Exchange" for my rims. If you happen to be in Canada, check out TireTrends.com - they have reasonable prices for 17" & 18" M3 Replica rims...
4 - Depends on the aspect of the rim... with my OEM's I use 225/45/17 on the fronts and 235/40/17 on the rear (as I recall), and the REPLICAs have 225/45/17 all around. If you use stock BMW sizes, just check your owner's manual - mine has listings for 16 & 17" rims (and maybe 18 - can't remember).
The only other thing you might want to look into is a set of spacers - these are just metal discs that fit between your rims and where they actually bolt to the car and serve to push the rims out a little bit - I find rear rims on most BMWs (except M3s) appear to be sitting in to close - almost look like they are tucked under the fender too much ....
cheers,
--GTC
