What they said about junking your old system may not be completely necessary. You could simply order a motherboard/cpu bundle from somewhere like mwave.com (I just ordered a MSI-6309 and Celeron 600 for $171), add an Enlight 7237 case for $47 (250W) or $62 (300W) and then throw your peripherals in there (ram, hard drive, floppy, CD-Rom) keep your existing mouse, keyboard, speakers, monitor, etc. That is basically what I am doing because I have severe budgetary constraints. Not sure if you have integrated sound and or video in your gateway, but there are cheap options for those as well. You should be able to find a sound card for around $30 or less, (and the MSI-6309 does offer built in sound, but I prefer the 3D sound affects) and Bestbuy currently has voodoo 3 3000 video cards for $79.99 and the V3 2000 PCI for $69.99. These are all (IMHO) bang for the buck bargains, and for some of us they are necessities

! I am just trying to decide now whether or not to spring for a Geforce 2 MX video card or just go for the reliable, fast for the $ Voodoo 3 3000 ( I already have 2 Voodoo 3 3000 PCI cards in two of my other machines, and have been very happy with their performance/dollar ratio).
Anyway, good luck whichever way you decide to go and come back often and let us know how it goes. I know I have learned a ton from the good people on this board as well as Hardwarehottalk.com.
Chuck