Not to be a bummer, but it would basically be pointless to create a 3G swap file, and it definitely won't turn your "old very old" PC into a rocket. A few years ago the rule of thumb regarding the size of the swap file was to make it "twice the amount of RAM", which in your case would be 128 Mg, but currently, where systems with 256M, 512M, or even 1G of RAM are common, that guideline doesn't really apply. (Having a 1G or 2G swap file is just a waste of disk space, since the swap file almost never gets used when you've got half a gig or a whole gig of memory.) Personally, I'm running one box with 512M RAM and another with 1G RAM, and the max usage of my swap files (both are 256M) has been maybe 30% if I recall correctly.
Anyway, if you want to improve the performance of that old PC, the best solution is to take mikshaw's advice and boost the RAM to 128M or 256M, or, ideally, the max the PC can take. Even if you do that though, if your CPU is as old as I think it might be (I'm guessing maybe you have a Pentium 133 or 166, like I used to have) your chip could turn out to be the limiting factor. Adding RAM will definitely give you the biggest bang for your buck, but even so it would be wishful thinking to expect that a PC that was built in the 90's could adequately compete with a system built in the last 6 to 12 months.
Perhaps it would be worthwhile to buy or build a new PC, and install that hard drive in it as a secondary. Good luck with whatever decision you make -- J.W.