First Time Kegging - Used Setup, need to refill?

Alright, guys, be gentle... I'm venturing into the wild world of kegging this weekend, and I bought a system off Craigslist. It's a 5lb tank and at room temperature, the gauge is reading 750psi. Do you think I'll need to refill it to force-carbonate my 5gallon batch? I don't want to get halfway through it and run out - also need to consider having enough gas to dispense... I'm not a gas scientist, I'm sure that tank PSI isn't an accurate indicator of anything.

Your best bet is to weigh the tank. Somewhere stamped on the tank is the tare weight (the weight of the empty tank), may be marked as "TW" or something to that effect. Weigh the tank, subtract the tare weight, and that's how much gas you have left. Granted, what you need to weigh is the tank only, no regulator or anything attached. If you take off the reg, make sure you put it back on properly and leak free. As long as there is liquid CO2 left in the tank, the high pressure gauge isn't much use.

Granted, I guess it could tell you the temp without a thermometer...