Can you read off the side of the PSU what the 12v rail is and what other components are in the system?
To be honest, spending a ton of money on a processor and possibly a psu for this upgrade is driving down the wrong path.
You would be much better off to save up and try to build a budget box of your own, and take parts from the Dell (cd rom, floppy, keyboard, hard drive, mouse, monitor, any PCI cards, etc.)
For example, if you jump up to, say, a super-cheap AMD X2 socket AM2 system, you will get a considerable improvement in almost every area.
Here's a barebones system:
http://www.newegg.com/Product/Product.aspx?Item=N82E16856167010
$79.99
You get case, motherboard, power, audio, video, LAN. You have a hard drive and a cd-rom, presumably.
Then you need CPU, RAM.
CPU $35 AMD 3400+
http://www.newegg.com/Product/Product.aspx?Item=N82E16819104005
RAM $35 2GB Crucial DDR2
http://www.newegg.com/Product/Product.aspx?Item=N82E16820146571
AFter MIR
So for $150 you could jump to an AMD 3400+, on the AM2 platform, get 2GB of ram, improve your PSU, and get a new case. For an extra $30 you can get an AMD X2 4000+ instead.
If you're a gamer, your only other expense will be a PCI-E video card.
Newegg has many barebones systems. Here's a nice one for about twice as much as the MSI:
http://www.newegg.com/Product/Product.aspx?Item=N82E16856110061