Do you mean PCI or the new PCI-Express? Manufacturers stopped making the old PCI cards ages ago, as all PCs now use the AGP port on the motherboards instead.
However I'd doubt you'll get one for £100 that will run well with HL2 and D3, but should be playable (does depend on the rest of the spec of the PC too, though).
Card-wise, your best bet is to either get an nVidia GeForce FX5900 or an ATI Radeon 9800SE.