General wifi questions

is wifi generally considered faster than 3g? How much more?

802.11n equipment will generally report connections in the 130-150 Mbps rated range. 3G a minimum data rate of 2 Mbit/s for stationary or walking users, and 384 kbit/s in a moving vehicle, but various rates are sold as 3G by carriers so this will vary from carrier to carrier.

-when using the iPhone on a wifi network does it use any data allowance?

No.

is it worth getting wifi at my house from my CPU to have a faster iPhone and less data usage from 3g? (considering dropping the unlimited data plan if so)

Yes. Since iphone 4 supports N, make sure you set up an N speed router to take full advantage of the new phone.

Your 3GS only supports up to G speed, the new phone will have a N speed chip(802.11b/g/n Wi-Fi (802.11n 2.4GHz only)).

802.11n is the speed, 2.4GHz is the channel. There are are number of dual channel routers on the market(2.4GHz & 5GHz) capable of broadcasting on both channels simultaneously or a single channel router can be set up to broadcast on either channel.

just spoke to an AT&T rep (this confirms AT&T website)...

to use any iPhone, you must have both voice and data plan.

You can't drop data plan and keep only voice plan.

No. Only the iphone 4 will have an "N" chip. Software can't change hardware 🙂.