I just recently built my own computer and it's been a success so far. Now I have a relative that wants me to do an upgrade for him.
This is what he currently has...
Raidmax ATX-268WSP Computer Case with 350 W PSU
Abit KV-85 Socket 754 VIA K8M800 Micro ATX AMD Mobo
AMD Sempron 64 2600+ Palermo 800MHz HT Socket 754 Processor
Corsair 512MB 184 pin DDR SDRAM DDR 400
Maxtor Diamond Max 10 80GB 7200 RPM HD
Basically he's not asking for much. He just wants a computer that will play online videos (baseball games) at a good speed. His current build is slowing down and the video feed is very choppy. He also net surfs. He doesn't play games. Just videos like youtube and live online feeds.
His budget is $200...$300 max. He wants a better processor. A cheap but good video card for viewing videos.
You really have to ask yourself if it worth doing anything with the old unit - HD could fail, PSU could be on it's last legs. It would not make any sense to put more money into it other that a stick of ram (see below) and clean up the hard drive.
1st cheapest fix (and the only one I would try) is to just get some more memory for the unit. The MB supports up to 2 GB of DDR400, but even adding another 512 is a good start (if there is an empty slot - throwing out what you have in order to increase ram is just not cost effective in this case). Online site Newegg has them now at under $20.
2nd thing is to do some "free" HD maintenance. Clean out unused software, clean out cookies and temporary files in the internet browser. It's amazing how this internet garbage can fill up a hard drive. Do a disk defrag.
If you really want to proceed further, it's a 2005 era MB so your restricted to only AGP video cards (4x / 8x). You might luck into a new one for under $60. You don't say what operating system but I expect XP? Make sure it is compatable with the card.
Most logical solution however is to just go shopping for a $300 computer (new or used) and reuse the old monitor. This "new unit" should have the same OS as the old unit otherwise you might run into software compatability with a newer OS. That equals more $.