wireless USB adapter. definitely. because it's a lot more portable, should you ever decide to change PCs, or bring it with you to work, or ever get a laptop, etc.

is it just as fast as the PCI card? (assuming that the USB card and PCI card are both 802.11g)

same speed. the network is going to move at a MAX of 11 MB/sec of 54 MB/sec, the way the card is installed has no bearing on this whatsoever.

well, just how fast can a USB port transfer data?

USB 2.0 can transfer data at the rate of 400 Mbps (that is approximately 50 MB/s). Which means I should probably get the PCI card, unless that transfer rate is lower.

Even with the slowest PCI (if I'm reading this correctly) is way faster than USB, though less portable. PCI transfer rates: http://www6.tomshardware.com/motherboard/20040514/e7505-chipsatz-03.html

Wait this is more accurate: "With up to 130 MB/s transfer rate in both directions the same basic performance is provided on a notebook as a PCI card would offer on a desktop computer." Taken ...