Regular AirPort (802.11b) is limited to 11Mbps, AirPort Extreme (802.11g) goes up to 54Mbps. Your internet connection is probably limited to about 1Mbps... so no, you probably won't notice any difference.
Your machine will only accept one or the other, but you could go either way for the base station.
The original airport is more than fast enough for most high speed internet connections. I doubt you would see any difference with Airport Extreme for internet browsing.
Extreme will be faster for transferring files and data to other computers on a local network. If you do this a lot then Extreme will have an advantage. if not then save your money and stick with the original version.
I just installed Airpot Extreme in my 1 Ghz G4 12.1" Powerbook. I used a WAP through my router rather than an Airport base station and am pleased with it so far. I do however notice that the further I get from the WAP, the slower web surfing becomes. I assume this is normal behaviour.
They're slowing trying to fase out/ get rid of 'b' cards - slowly... but most of the networks out there are 'b' anyhow. I'm in no rush to get a 'g' rated card - although they're all backward compatible. But will be forced to if I do get the 12" PB. No PC slot will not allow me to do any other - unlike the 15" alPB - which has a slot. But the internal is Xtreme only. No real $$ delta either... you can find a used A.Card out there, but there should be no real perf difference.
Your throughput speed tends to be bottlenecked by your ISP (i.e. 1 Mps), so the biggest difference you'll likely notice is when you use Airport to connect to another Mac (e.g., copying folders while streaming from iTunes).
In terms of wireless speed relative to an ethernet connection, I use the old Snow ABS on my iBook and it is just as fast as my hardwired connection on my G4.