I'd just head to the local store and probably pick one out. Maybe stick with the Linksys brand, make sure it supports Windows 7 and uses an interface that your PC has, either PCI or USB. USB will be easier but probably not as quick. Regarding the Xbox... yes you'll need one for it too unless MS just started including them. PS3 and Wii have them built in, but I think 360 comes with wired only by default.
I generally say to stick with the same manufacturer, although you don't have to. Since your using a G speed router the USB will work just as fine as a pci one for normal web browsing. If you will be gaming then I would look at a PCI version.
Here's what newegg has: http://www.newegg.com/Product/Productcompare.aspx?Submi...
One has speedboost the other doesn't, upto you if it's worth it or not.
Cards from other manufacturers should work just fine, although maybe not extras like speed boosting. Not sure on this though.
I recommend neither PCI nor USB. Get a wireless Ethernet bridge instead, less headaches. Same thing for the XBOX. In fact, you may be able to SHARE it w/ the XBOX if both are located in the same general vicinity (yet another reason among many I love using a bridge).
I prefer dd-wrt compatible routers (dd-wrt is third party firmware). By coincidence, your WRT54GL already supports it, so you *could* convert it into a wireless Ethernet bridge and maybe buy another router. Not saying that’s the best decision, but just putting that out there for consideration.
The least expensive deal I'm seeing at the moment is for the Linksys WRT54GS-RM @ Mwave ($25 shipped, although recertified):
Has been as low as $31 shipped/after rebate in the past, so a bit high priced at the moment. But compared to most other bridges, still a lot cheaper, and you get a lot of features. Wireless N, WDS bridging, wireless client bridging, wireless repeater bridging, two LAN ports (presumable extendable w/ a switch), even multiple SSIDs for private and public access. Not that all those features would be applicable for your needs, but should you decide to use it in some other capacity later, you have a lot of options.
Anyway, as I said, I use dd-wrt compatible routers and wait for the best deals. Sometimes I’m able to get them for $15-20, but that takes some patience. Personally I believe the best value is the WL-520GU. Brand new, terrific w/ dd-wrt (or tomato) installed, exceptionally strong signal for its class, USB port, will accept all but the largest dd-wrt firmware (larger == more features), etc.
Or I suppose you could grab another WRT54GL if you found a good deal on that unit and install dd-wrt or tomato.