Yes--bobkn is right, I do believe.
Normally if you went with i7, you would buy 3 sticks and run them in triple channel, so you would have 3 gigs, 6 gigs, or 12 gigs, depending on whether you bought 1 gig sticks, 2 gig sticks, or 4 gig sticks.
On an i5, it would typically be 2 sticks or 4 sticks; either 1 gig each, 2 gigs each, or 4 gigs each, for a total of anywhere from 2 to 16 gigs.
As far as I know, you can run 3 sticks in an i5, but I assume that would mean running in single channel mode. I would guess that if you had to choose, 6 gigs single channel mode would be preferable to 4 gigs in dual channel mode.
Sergio: that Intel motherboard you are looking at is detailed here:
Intel® Desktop Board DP55WG - Intel® Desktop Board DP55WG support
It will support these RAM speeds: 1600/1333/1066 MHz DDR3 SDRAM, with a maximum of 1.6 volts.
Since you won't overclock, 1066 MHz is all you need and that will save you some money.
Here is more detail on memory. It says single channel and dual channel are both supported. There is a list of specific brand names and part numbers that will work.
Intel® Desktop Board DP55WG - System memory
FYI: within 3 or 4 months, Intel is supposed to release "Clarkdale" processors, which will be the 600 series. Several of these will be in your price range.
Clarkdales will be i5, socket 1156, and may use a different motherboard chipset than the current Lynnfields. All Clarkdales will have video ON THE PROCESSOR. All are dual core with hyperthreading and Turbo.
670, i5, socket 1156 3.46 GHz
660, i5, socket 1156 3.33 GHz
661, i5, socket 1156 3.33 GHz
650, i5, socket 1156 3.20 GHz
In turbo mode, which is single core only, they can clock roughly 10% higher in each case.
The 670 will be around $300. The others $200 or less.
So the advice is, don't buy until the last minute. These Clarkdales may be available at that time.