"Can Windows XP really multi-task?" - not that much better than the old DOS "Print" command actually. There's still only one CPU in most systems.
Presume you mean a Pentium III at 866MHz. Windows already buffers the print queue and I don't know if it is possible to physically increase the built-in buffer on the 720 as you were sometimes able to do on older printers. My HP710C is d*nd slow which is why I use the old HP520 much of the time. But it's marginally faster from the XP/256/P4/1.6 than from the Win98/256/P3/800. I know that you can still buy in-line printer buffers, but they are expensive and I doubt whether they will solve your problem. I don't know how happy XP is on a 866MHz machine - have you tried turning off some of the XP bells-and-whistles? Is it only with complex images or does a simple text file print faster? Have you got a lot of unnecessary background processes running? Are you using the latest printer driver for the 720? I suspect that the problem is slow rendering of the print image and too many things happening at once which only a faster processor/bus will correct.