good luck finding snows.. esp in 17 inchers. i was in the market recently and actually ordered the tires and the rims, but it turned out they goofed on their end and misstagged the tire and sent me the wrong kind. and then they told me that ones that m3s are out of stock everywhere, thanks to the big snow storms! so i balked out on the deal, and i'm still stuck w/ my summers..
anyway, my best advice would be to limp by this winter.. (unless you can find a set of rims/ tires for a good price, which will be hard) and shop early for next winter, like around oct. buying 16 inch rims will save you a bunch on the tires, as 17 inch snows are quite expensive. one cavet to this is the fact there there is only one 16 inch rim that will fit your 330i, which is AT italia type 5 rims. ugly if you ask me, but i'm thinking of getting em for next winter. (going w/ 16s would save me around $300 per winter tire set) tirerack.com seems to be the only place that carrys them, and are currently out of stock until end of feb. it's 139 per + shipping.
xmas63 is absolute right. get the wheel/ tire combo as mounting/dismounting costs over the life of your first winter tire set will probably equal if not cost more than getting a dedicated winter rims. besides you dont' want to mess up your nice rims in the winter, not to mention possible damage during the mounting/dismounting process.
and finally DON'T regret trading up. it's only a tire issue, i can promise you that
in the mean time, enjoy making doughnuts in the parking lot.. just don't try to drive it home afterwards