hmm... you're upgrading from what? DDR2? what's the issue that you wanna perform an upgrade?

Unless you're getting the new generation X99 or Z89 or Haswell-E, no point getting DDR4. Will be expensive as hell too.

Kdr31, you're bringing up a good point ... In addition to the common assessment of performance and cost, there are also additional considerations that are important to how a person values their rig. Based on my experience, most people want a small rig, that's also quiet, in addition to being powerful enough for them to game as well as do their workstation tasks.

So low noise, and mITX criteria becomes important in addition to being powerful and cheap.

Small form factor build are cheaper?

Small form factor build are cheaper?

Generally, for the same level of performance, the smaller form factor build will be slightly more expensive, due to the motherboard, and casing.  For example, an iTX motherboard will generally be more expensive than an equivalent mATX motherboard, and nice iTX cases are slightly more expensive than mATX cases as well.  Also, the iTX build will normally be more difficult to cool, and this will need more careful selection of components to maintain the same level of performance and/or reliability.