Yes, you can.
You can use the WRT54G with Airport Express. I've seen posts on here about that. You will need to use a third-party firmware to enable WDS, I'm afraid I can't help you find that as I dont know where to look!
If you dont need wireless printing/Air Tunes, Linksys sell the WRE54G which is a wireless expander for the Linksys router. I just bought one of these off eBay for £37, which is less than half the price of an Airport Express, and doesnt need third-party firmware to run it.
I'd be wary of the firmware which can increase the gain of the router, as it probably wears out the components in the router much quicker. Similar to the stress you put on a computer when you overclock.