My goal is to send media files over the G network without B devices slowing everything down. I heard (and maybe you can confirm) that if B devices are on a G network, everyone's traffic slows to B speed.
I figured it would be easier to have separate B and G networks.
Nothing is simple when you're mixing media like this. Nor will it be cheap. Reconsider your expectations of what you are doing, and the price point you want to do it at and see if you can get inventive. You're trying to make a silk purse out of a sow's ear. If you want to stream dedicatedto video and/or multimedia, then 802.11a will provide a lot less cluttered band, plus exclusive bandwidth.
There's no need to share existing file sharing and internet services on the same wireless connection...none what so ever.
Plus, you have a limited budget, but something like this isn't for limited budgets. Take this into consideration: 802.11b and g both have limited bandwidth. As soon as you bridge both connectins, one connection is going to have to slow down to communicate with the other. You're wasting bandwidth and money in the 802.11g. Just keep your entire wireless network consistent. You can still run a wireless network for laptops and the such, but do not run your multimedia services on the same topology - you risk picture degradation and a large loss in quality.
Having the separate channel that really can't interfere with any other devices is a good reason to get 802.11a.