Either a USB Wi-Fi adapter or a PCI card should work with your PC. USB has the advantage of not having to open your PC to install it. If your PC does not have high-speed USB (2.0), you would be better off with a PCI card, as the USB would be a bottleneck otherwise. The simplest way to check if you've USB 2.0 is to plug in a USB memory stick. If you get a message saying "Your USB device can perform faster...", then it's the older USB 1.x.
The best PCI card to go for would be one with a small wired aerial, such that you can place the aerial on top of the PC or desk. Edimax has a PCI card with a wired aerial. If the aerial attaches directly to the back of the PC, this will greatly reduce the signal from my experience.
If you get a USB Wi-Fi adapter, get a USB cable extender unless the card comes with one. This will let you position the Wi-Fi adapter on top of the desk. Again, like the PCI card, you'll get better reception with the adapter placed on top of the desk/PC instead of plugged directly into the USB port behind it.