From Anands review of the mac mini
Memory installation is pretty easy, considering it uses regular desktop 184-pin DDR DIMMs (you still need to buy Mac specific memory - G4 or G5 memory will work because the motherboard is quite sensitive to the SPD on modules). The first thing I noticed was that the left latch on the memory slot can't be extended all the way. Apple definitely did their best to not waste a single centimeter in here:
The default memory that ships with the Mac mini comes from Hynix and features no heatspreader (not that one is needed, as there are obviously no memory overclocking options on the machine). Unfortunately, all of the Mac memory that we had in the lab came with heatspreaders, but to our amazement, our OCZ G5 modules actually fit in the system. It was a tight fit as you can see below, but it worked
From the dual G5 review:
There are a total of 8 DIMM sockets on the motherboard, requiring 1GB modules to meet the 8GB memory limit the system supports. The memory, as I've mentioned before, is the same DDR400 that you use in PCs, but the motherboard is quite picky about the SPD programming on the modules. The modules that the board supports are also quite slow, with very conservative memory timings (3-3-3-8). I don't believe that I've ever tested anything that slow on a PC before. Luckily, you can get G5-compatible DDR400 from more sources than just Apple; OCZ was the first to send me some compatible sticks, both 512MB and 1GB versions that worked perfectly. Other manufacturers also have Mac-lines of their memory.