When it comes right down to it, you interact with your computer through the keyboard, mouse and screen.
Screen size
For laptops, screen size is an important issue because it usually affects the overall weight of the unit as well. Smaller screen size usually means lighter weight and better mobility. Larger screen size means you can fit more on the screen. Balancing these two issues is a personal thing, and will depend on how much "mobile" use the laptop will get.
For desktops, go for the largest screen you can afford. Remember Windows is now capable of running many programs at once, and having a big screen means you'll be able to see more than one at once.
Keep in mind, this will be the main thing you'll be looking at, so bigger is almost always better.
Keyboard & Mouse
When choosing a keyboard and mouse, don't be too quick to jump on the Wireless bandwagon. Wireless mice have a short delay before the cursor starts moving, and they do use batteries which need to be replaced after about 6 months (that depends on the model and usage of course).
I'm a big fan of a standard Wired keyboard and mouse, there are no batteries, you're always getting the best performance, and ergonomically, they are in most cases identical to the wireless devices.
You can spend a bit on getting comfort curve keyboards and fancy shaped mice, but I think that the standard shapes work pretty well.
Dont get caught up in a keyboard and/or mouse with heaps of extra buttons, the chances are that after an initial fiddle, those things will just get in the way in the future.
Visit the iseek store in Nairne, and talk to the friendly staff, they probably spend as much (if not more) time in front of the computer as you!