Greetings,
I am pondering an upgrade of my computer as well and have spent the past month trying to bring myself up-to-date on current technology. I bought a book on the subject, "The Absolute Beginners Guide to PC Upgrades". I've found it very helpful.
One problem I think you may face is the size of your computer case. I believe the Aptiva has a small case. I strongly suggest set the mid sized ATX case as your standard.
Toms Hardware guide has a very good article that describes how you can build your own computer for around 950 dollars. I am looking down this path. I plan to use my relatively new 40 gig hard drive, CD, and CD ROM as well as my sound card, vidio card, and modum. Oh, I will likey by a backage that includes a new case with 300 watt power supply, mother board, CPU, and 128 megs of RAM. My 192 megs of RAM will go nicely with that. This package includes a 1 gig CPU 266 bus (get all the bus you can). This package goes for about 425 US dollars. I plan to get this from a site I found called MotherBoard Express found at
http://www.motherboardexpress.com . I have no connection to that business other than being a customer.
I think I've been too long winded so will say best of luck and God bless.
From a servant of Jesus Christ