I recently purchased a used MacMini3,1 (Late 2009) with a 2.26 Intel Core 2 Duo processor, and 2 GB of memory. The seller shipped it to me with Snow Leopard installed, and it ran smooth and fast. I just upgraded it to Lion and it's very sluggish. Would adding more memory fix this, or possibly upgrading to Mountain Lion make the computer run better? I installed Yosemite and it was unusably slow. I realize the computer is old, but does anyone have a solution?
The 2GB RAM is the biggest bottleneck here, no matter what you use the Mac Mini for. No recent Mac has been shipped with less than 4GB installed. Although memory management has gotten much better from Mountain Lion > Mavericks > Yosemite, you need to increase the RAM to at least 4GB.
Secondarily, you may be able retrofit a SSD to replace the slow internal HD.
I and my staff upgraded >200 Mac Minis of this vintage from 2GB to 4GB a number of years ago, in anticipation of upgrading from Snow Leopard to Lion & then eventually Mountain Lion. It enabled us to push the use of these computers another 2 years or so.
(Disclaimer: We are institutionally satisfied, (and I am personally satisfied,) with both of these company's products, and have no vested or financial interest in either company.)
Opening up a Mac Mini for the first time can be a daunting operation. You're only going to want to do it once if you can help it, so plan ahead and upgrade everything at once.
Maximum RAM for that model is 8 Gb. Maximum hard drive space is a total of 2 Tb split between the two internal SATA buses (the one used by the stock hard drive, and the one used by the optical drive).
I would recommend upgrading the RAM, replacing the hard drive with the largest SSD you can afford, and seriously considering replacing the optical drive with either another SSD or a second hard drive. You can always attach an external optical drive via USB or FireWire when you need it, and optical drives are so slow compared to hard drives that the bottleneck of the USB or FireWire interface won't make much difference (whereas you'll see significantly faster data transfer speeds reading or writing large files to and from a hard disk connected to the second internal SATA bus).
For example, depending on your particular use case, you might put the operating system and applications on a small SSD, and large files you're working on frequently on a larger mechanical hard drive, both inside the case, and connect an external optical drive as needed.
Edit: Other World Computing sells the brackets and cables needed to replace the internal optical drive with a second hard drive or SSD.
Two weeks ago I upgraded same machine albeit in a form of laptop — White Unibody Macbook with Nvidia 9400.
What I did was:
Added 8GB of memory. Buy two 4GB DDR3-1066 SODIMM sticks. System controller/firmware have problems with handling memory speed above that. You might look buying second hand or buying only one module of 1066MHz and another one can be 1333 since are they easier to find and usually cheaper. Having working 1066MHz stick present will command 1066Mhz for the second one. Many 1600MHz modules do not have predefined settings in its SPD chip for handling 1066MHz frequency but big names usually do. Look for Hynix, Samsung, Crucial and the likes. For 1066MHz module(s) recommendation going for big manufacturer still applies.
Replaced optical drive with so called Optibay. Here is how you do it for you Mac Mini. Optibay in this case is a name for a converter from miniSATA to SATA in form of optical drive) — eBay have plenty of them. Make sure it is 12.7mm SATA-SATA without handles i.e. not for Macbook. It will look like this
Installed cheapest big brand 128GB SSD into Optibay for system & apps and designated HDD as storage area for music, video and other data. I haven't seen need for bigger SSD you might think otherwise. Also HDD can be replaced with one up to 2TB if you need it.
Installed Yosemite. It is free and fast. Nvidia 9400 chip helps with flash and h.264 video decoding so I think this machine will last another maybe five years for moderately heavy use. Like no video editing (it is hard to predict technology though)