Well your broadband definitely won't be at 54mbps (more like 4mbps) -- so from that perspective the wifi will be more than adequate.
however, that's assuming a perfect wifi connection. Other factors, like distance, walls, and interference (from your cordless phone, for example) will cause slowdown of the connection.
I'd suggest you move your laptop to where your PC is. If it consistently runs at the same speed you expect your wired PC to, then your PC should be fine in wireless mode too.
Here are some examples for you:
http://www.ebuyer.com/UK/product/45622
http://www.ebuyer.com/customer/products/index. html?rb=22936378436&action=c2hvd19wcm9kdWN0X3J ldmlld3M=&product_uid=107691
If you're confident about fitting the PCI card inside your computer (there's not really anything to it), then I'd suggest that over the USB adapter.