N is the new wifi standard and backward compatible with a/b/g standards.
Its not all about the data transfer rate, it is also about the power of the coverage ie the are coverage -think of it this way - b was quicker that g, but the signal strength was weaker and so it had poor coverage, so in theory over a longer distance and through obstructions (stud walls etc) g is quicker.. then g+ came along which gave greater range before it turned to draft n (150Mbps) n (150/300Mbps) and n+ (300Mps, but at a greater range). So if you get an n card it will work at g speeds - but think g cards are well on their way out (ie not being produced) and will lack the software update to cover newer encryption.
Also internal (PCI or PCI-e) is better than USB, as USB has limitations on the power allocated to the USB ports, but it is rarely noticed as most people don't use all their USB slots and the router is usually within 100m of the PC (albeit with some obstructions).
I have been programming on computers since the ZX81.
I am an apprentice trained Electronics Engineer with qualifications to back it up.
I have been repairing computers since 1996.
Yet to some people I still know nothing...