Just ran into the same problem with an R310. Took quite a bit of digging but the link to the R310 Tech Guide helped. The 1GB UDIMM uses 128Mb x8 chips which is why it works. The Samsung sticks they keep sending you use 256Mb x8 chips and as you found in the Tech Guide the R310 won't work with 256Mb chips. I ran into the same problem using some memory we pulled out of an R710 server but they were 256Mb x4 chips so it was a double no-no.
Here is what I've found regarding memory. The actual DRAM chips on the memory module have differing numbers of lines. This is also referred to as the rank width, e.g., x4, x8, x16. The DRAM chips are also different sizes, e.g., 128Mb, 256Mb, 512Mb. The easiest way to explain ranks is to think of each rank as a virtual DIMM. So a single rank module is just that; a single memory module on a single physical DIMM. Dual rank memory actually has 2 memory modules on a single physical DIMM (it's deeper than that but this is the easiest way to visualize it).
So the bottom line is that the R310 will only accept DIMMs that are made out of 128Mbit or 1Gb/2Gb chips that are x8 devices. Or at least that's my story and I'm sticking with it until Dell tells me otherwise; e-mail to them is out and I'll let you know if they say something different.
Link to Hynix part: http://www.hynix.co.kr/datasheet/eng/module/details/module_20_HMT112U7BFR8C.jsp?searchType=2&menuNo=1&m=1&s=1&RK=20&RAM_NAME=
A link to an excellent briefing from Intel:
Slide 13 is the key, but the entire package is very good depending on your level of geekdom :)
Update: Confirmed with Dell except that I was wrong about requiring 128Mb and smaller chips. The Foxhollow platform does support 1Gb and 2Gb chips. See here: http://cache-www.intel.com/cd/00/00/43/26/432686_432686.pdf