Newegg has the Rosewill RNX-N150PCx 802.11b/g/n PCI 2.2 Wireless Adapter for $17 - $5 off with coupon code EMCNFND68 [Exp 4/8] = $12 with free shipping. If you're completely out of USB ports or if you just don't want to use one, this should stand in just fine.
Would there be any performance loss using this PCI card over a PCI express wireless card?
Considering your PCI bus operates at GBs/s and wifi is in MBs/s, I'd say no.
"If you're completely out of USB ports or if you just don't want to use one, this should stand in just fine. "
lol