In general you cannot run both B and G on the same access point at the same time. There are access point that either have 2 radios or try to simulate 2 radios by switching back and forth quickly. You could simulate this with 2 access points on different channels.
The key issue is that B and G encode the data differently even though they use the same frequency. Generally it will not slow down it just will not work. All depends on how the router is configured. Some will stay with the first protocol they find other will always switch to B if a B device appears. Some routers will only run what you configure them and not change.
Your cheapest option is to use a second access point/router. Some of the new N routers are "dual band". Although they will not run B and G they will run N and either B or G at the same time. They have 2 radios one at 2.4 and another at 5.0.
A final note no matter what you do every device you add is sharing the same bandwidth so it will always slow down to a point.
I've got a WRT54G Linksys wifi router, and am using WEP 128 and MAC address filtering.
I've got a seldom-used computer connected through a G bridge (a Netgear WGP5606 print server with four ethernet jacks as well as a USB printer port) and a Dell XP Pro laptop with a Broadcom DW1390 G card from Dell.
I also have a second WGP5606 that's almost never turned on, connected to my Blu-ray player (BTW - don't bother. BD Live is too slow to be useful, and burning firmware updates to CD seems safer to me than letting the player auto-download them).
Does bringing a B device onto a wifi network always slow down all the G devices or is that something that only happens sometimes? The B device is a Pocket PC that I can use as a wifi internet radio tuner in my basement theater - but if it unduly slows down the laptop or bridged pc, I might not be able to do this.