Sure you can find noname generic PCI G cards on the cheap but they might as well have "2002" stamped on them.
The N cards are outdated, expensive, unsupported and, well just plain suck.
USB dongles are not the ideal interface and almost never have multiple antenna adapters, if any at all. There are quite a few shoddy ones as well.
Meanwhile laptops get constantly updated models that work great and have excellent, comprehensive drivers. The cynic in me says most are pretty good because they actually have to work, often being the primary/only interface used on most laptops.
You have to look around (e.g. not in stores or newegg etc) but you can buy the minicards by themselves dirt cheap. Fleabay is the typical place but there are plenty of other sources if you want to avoid that.
Herein lies the problem...they only fit into laptops.
Thought I'd share my new found solution after dealing with the underdeveloped shoddy mess that is desktop wifi cards: its pretty much perfect for wifi nuts like me.
Minicards are just pci-e x1 and usb header stuffed into one tiny slot
PS, I may be a wifi nut, but I'm also a hardline extremist about running cat5 whenever possible, short single-home wifi lans are a bad excuse for lazy nerds!
(although its "ok" to have a couch/porch/whatever wifi in addition to your gigabit and long range private WWAN etc )