"I have 1.5 GB of RAM in this laptop, what's the maximum I can set my virtual memory to?"

You can set the virtual memory to as much as the maximum capacity of the hard drive.  However, there is no point to do so (if your PC has to swap 100GB of virtual memory to hard disk, it will be slower than a turtle).

The "unofficial rule of thumb" for virtual memory settings should be:

• minimum:  same as physical memory of the PC
• ideal:  1.5x the physical memory of the PC (I believe this is XP's default setting)