I personally use Kingmax PC2100 as when you read the reviews regarding this make of memory ( on the front of overclockers.com ), it runs very well on lower voltages and because a great deal of these systems use low power it makes a lot of sence, the Kingmax also has a low profile, some DDR have dimensions which are larger than others and can become the highest point on the system if no additional PCI cards are fitted. I would personally use 512 due to everything being onboard.. this amount of memory runs makes a very nice system.
With regards to what (SFF, small form factor ) I would suggest it would have to be Flex-ATX.. I believe that some mobos that if you run a lot of the components close together such as on the smaller MINI-ATX create additional heat problems and can be flakey and unpredicatable in every sence of the word.. in the small ATX based systems Flex is as small as I would personally go and Celeron is the processor I would use, the other processors just create to much heat.
If buying SFF.. buy a high quality make
With regards to the ITX platform I would suggest a ME6000 if you only want active cooling ie no processor fan ( 600Mhz Eden Processor, Embedded MPEG-2 decoder, 6 Channel audio, 10/100 Ethernet, USB 2, Firewire and TV-Out support / Takes DDR Memory ) or if you want more power look at the M9000 ( 933Mhz C3 Processor, Embedded MPEG-2 decoder, 6 Channel audio, 10/100 Ethernet, USB 2.0, Firewire and TV-Out support.
The reason I reccomend the M9000 over the M1000 is that the difference in performance is seriously marginal and more noticable imho is the increase in price, if you do decide to go for a M1000 ensure that you get a Nehemiah M10000 and not a M1000 with a Ezra-T core