Upgrades:
1. Memory - upgrade to 8gb ram ($71, just add another kit)
2. Backup drive - add another 500gb 7200rpm drive ($80)
3.
Griffin iMic USB sound card - $30
4.
D-Link Bluetooth USB adapter - $26 - add Bluetooth to use an official Apple Bluetooth keyboard or mouse, or even a Bluetooth headset for chatting on iChat or Skype.
5. Firewire PCI card - if you want Firewire for a camcorder or high-end scanner or whatever. About five bucks on ebay.
Accessories:
1. Keyboard
2. Mouse
3. Monitor (Dell makes fantastic LCDs for about 1/2 the price of Apple LCDs, using the same displays that Apple uses in many of their monitors)
4. Speakers (I recommend Logitech)
5. Card Reader (I've had success with 3.5" internal drives as well as USB drives, plus drives built into my Dell monitors)
Notes:
1. This board supports 45nm processors. E4500 etc. are welcome.
2. This board supports up to 8gb ram. Max the beast for $142.
3. Onboard audio works, but stops working if you put the computer to sleep and wake it. I recommend the Griffin iMic USB sound card linked above for $30, it works perfectly! I have one and love it. If you never put your computer to sleep, then you don't need one.
4. I highly recommend a backup drive. My suggestion is to use a combination of Time Machine (comes free with Leopard) and
SuperDuper ($28). Time Machine backs up your files hourly while SuperDuper clones the entire system. SuperDuper can create a sparseimage file which can be restored to a hard drive using the Leopard installation disc (just use Disk Utility) - it's similar to Norton Ghost or BartPE. Do this nightly or weekly. Combined, you have a full system clone plus a backup of your files up to the last hour.
5. Kalyway Leopard will work on this.
6. The motherboard has 6 onboard SATA ports, so you can add hard drives and optical drives up the wazoo. Personally I use 3 drives - a boot drive (500gb), a backup drive (500gb), and a scratch drive (500gb). I store my VMware virtual machines on the scratch drive - that way my Windows VM isn't reading/writing on the same drive as OS X, which helps it go faster. Not necessary but I run my VMs 24/7 and like the speed increase.
What you get from Apple for $800: (for comparison)
1. 2nd-tier Mac Mini
2. 1GB DDR2-667mhz memory
3. 120gb 5400rpm 2.5" SATA hard drive
4. Slot-loading 8x DVD burner
5. 2ghz Core 2 Duo (Merom - mobile CPU)
6. Extremely small form factor
7. Quiet operation
8. Intel GMA950 built-in graphics with single DVI port
9. 4 USB ports
10. 1 Firewire 400 port
11. Not very expandable (only hard drive, optical drive, memory, processor, and wifi)
12. Expensive to upgrade (requires laptop parts, often twice as much as desktop parts)