I would not want to spend more than $100 on it unless it is for something that could be used in a later build.
You could get a nice cheap case like the antec 300
Then you could get a nice psu so you can power a better gpu later.
These are both upgrades that could be used later.
Besides that you are looking at adding some RAM
512MB 168-pin DIMM
* Module Size: 512MB
* Package: 168-pin DIMM
* Feature: SDRAM, PC133
Looks like you have 2 ram dims with a max of 1gb total. Are both slots populated at the moment with 2x256mb?
If so you could get 2 of these:
Newegg.com - WINTEC AMPO 512MB 168-Pin SDRAM PC 133 Desktop Memory Model 3AMSD133-512M-R - Desktop Memory
$23 each. If you just have one dimm filled with a 512mb stick than you could just get one.
You could get a vid card like this:
Newegg.com - XFX PVT44AWANG GeForce 6200 256MB 64-bit GDDR2 AGP 8X Video Card - Desktop Graphics / Video Cards
But it needs a psu of at least 300w to run. So most likely a case as well in less you can find a slim psu but they are overpriced at best.
If you reused hard drives, keyboard, mouse, monitor and dvd drives from your Dell for another $300 to $400 you could get a nice system.
So upgrade route you are looking at $100ish for ram and gpu. Another $80ish for a case and psu. More than halfway to a new computer by then.