I have an HP MediaSmart Server (HP EX475, running Windows Home Server) for backing up several systems and sharing various kinds of files across my home network. It's a nice little server, with 2.5 terabytes of disk (4 drives, already upgraded from the standard single drive by me following HP's instructions), 512mg of memory, and 1.8 GHz AMD Sempron 3400+ CPU.
The HP MediaSmart Server as it came out of the box performs very well for home backup and file serving... however being both a performance car enthusiast and a computer nerd I'm of the opinion that you can never have enough power. So, an upgrade is called for. But, which one? Will it run too hot? Can the existing power supply handle it? What about the BIOS?
Enter the Home Server Hacks (http://www.homeserverhacks.com) site. They've already answered all of the questions so I simply followed their lead. Be sure to read the disclaimer - this will void your warranty. However, mine has been running flawlessly for several months, so the warranty was not an issue for me. Your mileage may vary; this is not something you should do unless you have done this type of upgrade before.
Their instructions are here:
As they recommended, I used an AMD 2.66 GHz LE-1640 Orleans 45w CPU. 45% more power... but also an increase in L2 cache from 512kb to 1 MB. The extra cache is a very important part of this upgrade. I also moved up to 2GB of DDR2 667 SDRAM (PC2 5300) memory. Note there is only 1 slot.
I used a corner of my work area to perform the upgrade. That's a Wistron motherboard, BTW. Very small and compact in a conveniently small case.

Be sure to use a static grounding strap so that you don't short your nice new CPU and memory. You'll also note I use an Ott-Lite to clearly light up the area. And of course as the instruction say, be sure to label all the cables as you pull them off.
Results: exactly as the website says. Same temperatures, faster serving of many types of files, and faster backup. A nice cheap upgrade with great results. Here's the CPU-Z (a more updated one than was used in the HopmeServerHacks postings above):

As to the HP MediaSmart Server, it's a great little box. Installation and configuration couldn't have been easier - anybody can do this in their own home. The backups are flawless and I can pull any file off the server at any time. I also have my entire music library on the server and regularly connect to it from any of several systems I own. Kudos to HP, and Microsoft as well for their very easy to use Home Server version of Windows.