If you plan to keep the old parts that seem to be working, then you really need to have a clear understanding of why you are upgrading, Adding a few new parts to an old car won't make it run any better, unless you add the parts that are actually causing problems. Same with computers.
How much memory do you have? If it's 512 Mbytes (or worse 256 Mbytes) then you probably want more.
What do you -use- the computer? What programs do you run? That makes a big difference on what sort of hardware you need. You can't choose between a pickup, an SUV or a Honda Civic without knowing what you are hauling. Again, same with computers. Given what you describe, I am guessing that you are not a serious gamer.
If it's "random crashes and hang", then it could be power or software (Windows cruft, spyware, and such). First try backing up your data that you care about (back it up and double check it twice, to be sure) and then reinstall Windows XP Home with Service Pack 2, which you can get on a CD from Newegg.com for $87.95. Best to install from a CD, rather than upgrading an older Windows XP across the network, because PC's connected directly to the Web running earlier versions of Windows can be infected with spyware in fewer minutes than it takes to download the updates.
Besides power supplies and windows software, another cause of old machines getting unstable is failing capacitors on the motherboard (aka mainboard). Diagnosing power or capacitor problems is not easy for the inexperienced. Easier (unless your time is worth far less than your money) to replace the box.
I have a 3 month old dvd burner, I am basically trying to use that and the speakers. Sounds like I need a new tower, power supply, motherboard, processor, video card/s.....new monitor. I want to build a high powered gamer, would like to spend around a grand. I have two hard drives that are freshly loaded with XP service pack 2...I am just going to try this....I hope I don't spend more than I have to, but I want something upgradeable, powerful, and fast. I may just end up buying one I found online.