Hello,
Bought a desktop today and need to buy a wireless card which one is compatible with windows 7
Anyone that says so on the packet - and that's not being facetious, it's a fact. Any manufacturer worth his salt will declare which systems it will work in. You need to decide how much you want to open up the case - possibility invalidating your Warranty - to fit an internal card or whether you'd prefer a little wireless dongle that plugs in to a USB Port.
To me, it's a no-brainer - the USB dongle for around £5 or $8 on e-Bay and fitted in seconds wins every time.
I would tend to go with PCI adapters for signal strength.
Opening the case will not invalidate the warranty -- no court would permit that because the original IBM design for the PC laid down that parts should be user accessible and the machine is not fully usable as a PC if you can't add parts occasionally -- that's why the power supply has to be in a closed box.