Power Supply's have a Form Factor. (Actually only motherboards do, but most Power Supply's follow the motherboard Form Factor)

The ATX designation determines it's size, shape, and factors concerning the various power cables coming out of it, (Spaghetti), plus other concerns.

An ATX power supply shape is generally close to 6 inches long, (15.24 Centimeters), 6 inches wide, and 3-1/2 inches tall.
(8.89 Centimeters)

It is the common size and shape used for 90 percent of the personal computers made, out there.

From what I have been reading about Evesham, they used typical power supply cables used for most ATX power supply's.

Basic specifications on the Evesham Evolution 3000 desktop computer,

http://www1.epinions.com/specs/pr-Evesham_Evolution_3000_evop3ghblulightmar04_PC_Desktop

Power Cables:

1) Don't know if the Power Supply will have a 20-pin ATX main power cable, or a 24-pin ATX main power cable.

In reality the point is moot, because new power supply's have a main power cable which will be a 20-pin connector, or a become a 24-pin connector.

To expound,

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20plus4

(If you scroll up the page, you can see a 24-pin ATX main power cable, and it's respective connector.

Scrolling further up produces a 20-pin ATX main power cable, and it's respective connector.

Color of the connector does not matter)

2) The Evesham Evolution 3000 specifications, state that an Intel Pentium 4 processor is used.

This means there will be a 4-pin ATX +12 Volt power cable, which plugs into the motherboard.

Example,

http://www.playtool.com/pages/psuconnectors/connectors.html#atx12v4

(Again, color of the connector does not matter)

3) There will be a few 4-pin Peripheral power cables,

http://www.playtool.com/pages/psuconnectors/connectors.html#peripheral

One connects to the Harddrive, one connects to the Optical drive, (Or optical drives. Spec's state just one CD/DVD drive is used)

4) There is a Floppy Disk drive power cable,

http://www.playtool.com/pages/psuconnectors/connectors.html#floppy

(Again, I'm going off of the specifications stated. Your Evesham may not have a Floppy drive)

To be continued in a Comment.