Bang for buck upgrade priority list.
1. Back everything up and do a fresh install of XP Pro and update to SP3.
2. This era will support at least 1Gb if not 2Gb. Some systems will require a bios update (quick & free) often found at the manufacturer's or the motherboard support site. Make sure it's the right RAM (DDR or DDR2?, pins, speed, Low/high density...) Use the Crucial.com scanner. Just bought 2Gb of Low density for an old P42.66 used on eBay for $36. Retail is often $90.
3. Get a video card matching your AGP slot, memory interface bit rate (64 or 128). Older nVidia (FX5500,5600,5700, 6200) or Ati cards w/ 256mb run about $20 to $35 used on eBay. Cards that have an extra power supply (molex) connector (nVidia 6600 and faster) might exceed your systems 250 watt power supply. If its a Dell the 250w psu is rated continuous which compares to a cheap psu peak rated between 320 and 340w which helps. If you have 2 hard drives, two optical (dvd/cd) drives... you are likely over your 250w budget especially with a graphics card that requires an extra power lead. Go to the Journey Systems paower calculator (easy). Just bought a used FX5500 for $20.
4. You probably have and IDE 5400rpm hdd. If it is old with say 2mb of cache, then a younger 80 to 120ish Gb hdd w/ 16mb can be had used for maybe $25 to $40 and will be a lot faster than a 2mb cache drive. Be ware of used drives though... it's good to know the drive's history.
5. If all that works out, consider a processor. Is it a 478 pin? If you have a 400fsb it might be to slow to mess with, but if it's a 533fsb you might run up to a 3.06Ghz, 512kb cache P4 Northwood (SL6PG, SL6S5, etc.). It's tricky figuring out which one will work. They run $25 to $30 (and up) used on eBay and sellers don't accept returns. I got the wrong one, but sold it for $12 more than I paid. Check the motherboard specs online and search to see what others have done regarding compatibility. Maybe a local tech can help for a small fee. If it doesn't work as soon as you plug it in, then it will take hours of techie time to sort and that will far exceed a reasonable budget on the system.
Having said all that, you might luck into an ok system for around $100, but you might chase your tail. If you're up for the challenge, that's great, but there is a risk of brain dammage... $#!+ :^( If you plan to run modern games or do video editing, you probably won't be satisfied. I just finished this upgrade and it does video edit, but compiling takes about 1 hour per 10 min of 640 x 480 video and it will not do HD. I have a jet screamin' quad core, but for basics, the ol' P4 does just fine. I could probably CraigsList off the ol' P4 (w/ nice 19" crt boat anchor) for $200.