What kind of wireless repeater/booster do I need?

Hi. I have a Linksys Wireless Router (Home Gateway, model number: WAG354G). I have my Xbox and projector on the bottom floor but the router is two floors up in the attic with too many walls and stuff in the way. Neither the xbox nor the router can be moved for a better signal to get between them. All other avenues of increasing the signal strength have made appreciable improvements but nothing effective enough. The last thing I can try now is to buy a signal booster/repeater to plug into downstairs. I've been looking all over and have no idea which one, for a reasonable price, would be compatible and effective in conjunction with my Linksys router. Can anyone help me and point me in the right direction? Has anyone else done the same with a Linksys system before? Fingers crossed. Cheers.