Here are very well written comprehendsive 3 buying guides CNET:

Home Desktop
Business Desktop
Laptop

The information from these guides will be far better than anything you're going to get here. What you'll get here is a potpurri of personal opinions from users of varying levels of expertise.

You don't need deploy Dell Optiplex computers in a non-networked environment - any computer will work as a standalone workstation.

You don't need the latest and greatest Core 2 Duo processor if you're running basic office needs (i.e. word processing, spreadsheet, database, email, presentations). A old fashioned 6 month old Duo Core Pentium D processor will be more than sufficient for your needs while still providing you an upgrade path to Windows Vista. You may find some excellent values for your office computers if you shop wisely in order to maximize your business' profits.

At home, it's a different story, especially if you plan on doing any hard core gaming. The speed of Core 2 Duo processors are advantageous for use in multimedia applications, especially when heavy duty processing jobs such as converting multiple WAV files to MP3, applying multiple filters to a large photograph in Photoshop, running a graphics intensive PC game, or editing video files. At home, the sky is the limit. The limiting factor would be your own personal skills. If you can't overtax your computer, then scale down your purchase accordingly.

Apple is ranked #1 for both desktop and laptop support, far exceeding any of the other PC brands. However, if you're not running Apple now, you'll also need an additional investment in Apple software, plus the cost to retrain any personnel on how to use Apple products.

As for laptops, if you constantly carry a laptop wherever you go, buy a laptop build to handle the rigors of travel. Because laptops are far more expensive than desktops, you're going to replace them less often - therefore, you want to buy a fast processor, an advanced screen, a big hard drive, WIFI, Bluetooth, and an big hard drive.

Here's the specs on a laptop I purchased from Dell 2 weeks ago for $1097 out the door:

Inspiron E1505, Intel Core 2 Duo processor T5500 (2MB/1.66GHz/667MHz)
15.4 Inch UltraSharp TrueLife Wide-screen WSXGA+, for Inspiron 6400/E1505
1GB, DDR2, 533MHz 2 Dimm for Inspiron 6400/E1505
Intel Integrated Graphics Media Accelerator 950 GM
120GB 5400RPM SATA Hard Drive for Inspiron 6400/E1505
Microsoft Windows XP Media Center Edition 2005
Free Upgrade to Windows Vista Home
Integrated 10/100 Network Card and Modem, for Inspiron
8X DVD+/-RW Drive for Inspiron 6400/E1505
Integrated High Definition Audio, Inspiron
Dell Wireless 1390 802.11b/g Mini Card (54Mbps), for Inspiron 6400/E1505
53 WHr 6-cell Lithium Ion Primary Battery, for Inspiron 6400/E1505
Dell Wireless 355 Bluetooth Module (2.0 + EDR),for Inspiron
3 Year Dell Full Service In-House Warranty