At the high end of complexity, you could get another Linksys WRT54G or
WRT54GL which is compatible with DD-WRT (i.e. not version 7), flash it
with DD-WRT, and then set it up for wireless bridge mode. This is
generally too complicated and tricky for novices.
A simpler solution
is to buy a similar device off the shelf. The cheapest one would probably be a refurbished Netgear WGPS606 from eBay or retail clearance. It's a bit dated, but does client mode bridging simply and reasonably well, and supports up to WPA-PSK encryption.
At the high end of consumer products, there's a new device from D-Link -- the DAP-1522. It'd be wasted in this application though with only a WRT54G to feed it.
Assuming your Linksys is compatible with DD-WRT, you have another option -- flash it with DD-WRT and get something else for the main router.