China Unicom uses UMTS 2100 if I am not mistaken. What I know for sure is that they use the HSDPA (among others) standard so if your modem can handle it you can get up to 7.2 Mbit/s download speeds (of course in practice it will be a bit slower). Where I live I am able to get between 3 and 4 Mbit/s download speeds, which is quite fast in China. Plus if you use a VPN you can increase download speeds as well (since it bypasses the you-know-what). There was talk of China Unicom upgrading to HSPA+ but not sure if this has happened yet (if it does speeds will reach 42 Mbit/s).
Basically for internet only access there are no contracts. It is a pre-paid system. I walked-in, handed over 500 RMB and was given a SIM card. Prices are like this:
60 RMB = 1GB downloads in whole of China + 1GB in Yunnan (so you get a total of 2GB). Then there are more expensive packages, but I can't remember the prices of all of them. I know the most expensive is 300 RMB and you get 10 GB of downloads. If you go over your allowance of 2GB for 60 RMB they automatically upgrade you to a higher priced package, so no need to pay for extra usage by the megabyte (which I think is a brilliant idea).
The above are not monthly charges...when money runs out you go back to the shop with your SIM card and recharge the money.
The only post-paid system is when you purchase a 3G SIM card that you will use for internet and phone calls (on your mobile). But the data allowance is lower and monthly costs are higher, so better opt for the simple pre-paid option.
Finally each time you disconnect, China Unicom sends you a message letting you know your data usage (so you can calculate exactly how much money you have left on your card).