Unless you know for sure, no guessing, that circuit can handle more than 20 amps it would not be a good idea to up the fuse rating. Unless you can have a pro come and test it to be safe I wouldn't recommend it.
In that old house what you need to be concerned with is the insulation on the wires. In my parents' old house in So Cal (with screw in fuses) the wiring had woven cloth insulation. The size of the wire (gauge) will dictate what size fuse you will max out at. If you go over that amperage you run the risk of overheating the wires and igniting the insulation on the wires - read: FIRE!!
I would be really surprised that a 120V circuit would go over 20 amp (12 ga wire). A 240V circuit could handle the 25 and 30 amp fuses you mentioned. Wid is right; get an electrician in there.
Scott
