Be careful, its generally only specific Atheros and Broadcom cards which support Access Point mode. Pretty sure none of the Intel WiFi chipsets have support for it.
The compatible Atheros chipsets should be:
AR5008:
AR5418+AR5133 (>= 2.6.27) AR5418 = DB 11n PCIe, AR5133 = 3x3 DB 11n
AR5416+AR5133 (>= 2.6.27) AR5416 = DB 11n PCI
AR5416+AR2133 (>= 2.6.27) AR2133 = 3x3 SB 11n
AR9001:
AR9160 (>= 2.6.27) DB 11n
AR9102 (>= 2.6.30, AHB) 2x2 SB 11n
AR9103 (>= 2.6.30, AHB) 3x3 SB 11n
AR9002:
AR9220 (>= 2.6.27, an AR9280 card over PCI) 2x2 DB 11n PCI
AR9280 (>= 2.6.27) 2x2 DB 11n PCIe
AR9281 (>= 2.6.27) 2x2 SB 11n PCIe
AR9285 (>= 2.6.29) 1x1 SB 11n PCIe
AR9287 (>= 2.6.32) 2x2 SB 11n PCIe
AR9003:
AR9380 (>= 2.6.36) 3x3 DB 11n PCIe
AR9382 (>= 2.6.36) 2x2 DB 11n PCIe
AR9004:
AR9485 (>= 2.6.39) 1x1 SB 11n PCIe
AR9462 (>= 3.2) 2x2 DB 11n
I highly recommend a 2x2 or 3x3 card, basically a 1x1 can do 150Mbit, 2x2 300Mbit and 3x3 450Mbit. Personally I am using one of these in 5Ghz mode, but it works equally well in 2.4Ghz (normal WiFi) mode too.
As for why I use USB network adapters, that is so that I do not waste gigabit ports in my network switch with low usage devices.
If you bridge several ethernet adapters together then it effectively becomes an ethernet switch. Yes it takes some of the CPU power, but as the Atom has plenty to spare and I am only using it for low bandwidth devices (one of which is the second port of my VDSL modem so I can telnet in and bring up the line stats, etc), its far better than wasting a full blown Gigabit port on the switch.
Also if you are only a couple of ethernet ports short, its a lot cheaper (about $3 a port if you get them from China via eBay) than having to upgrade an 8 port switch to 16 ports, which I would have to do as generally you are only supposed to have three unmanaged switches on an ethernet network. I already have three as its two routers (for two other WiFi networks) and an 8 port Gigabit switch. Obviously the routers I cannot swap out, the best you ever get is 5 ports on any router, so this was the only practical option.