There was a time when buying a MicroATX motherboard meant making a lot of concessions. Today, however, one need not lose more than a couple of inches off the width of the board. The GA-P55M-UD4 and P55M-GD45 are both solid P55 implementations that retain the enthusiast-oriented sensibilities and apparent overclocking prowess of their full-size ATX brethren within the confines of a smaller form factor.
These Micro ATX models are a little cheaper than their ATX counterparts, and they consume less power, too. The only real drawback: a two or three fewer expansion slots. I don't expect the extra slots will be missed by many. In fact, I'd wager that even most enthusiasts would be happy slapping in a graphics card and calling it a day. The UD4 certainly doesn't need much more in the way of accompaniment. Sure, you could plug in a sound card, but those with a digital receiver or speakers won't even need to go that far thanks to the ALC889A's Dolby encoding capabilities. And don't forget the hybrid eSATA/USB connectivity or the balanced dual-x8 PCI Express configuration that supports SLI in addition to CrossFire.
The only fly in the ointment is Gigabyte's apparent disinterest in providing users with meaningful BIOS-level fan speed options. The UD4's dearth of fan speed controls is embarrassingly dated for a cutting-edge mobo with an otherwise well-stocked BIOS, and Gigabyte would do well to study MSI's significantly superior approach.
Unfortunately, the MSI board has issues elsewhere. The lack of hybrid eSATA/USB connectivity is disappointing, especially since it's a feature available on MSI's mid-range P55-GD65. However, that doesn't irk me as much as the optionaland absentdigital S/PDIF audio output. I'm not crazy about the GD45's inexplicably slower USB performance, either.
To MSI's credit, the GD45 does cost a good $20 less than the UD4. The question you have to ask yourself is whether a better audio implementation, hybrid eSATA/USB, and a dual-x8 PCI Express layout is worth the extra scratchand the lousy fan speed controls. For me, the answer is yes. While the GD45 feels like it's been compromised to hit a lower price point, the UD4 really is a full-featured enthusiast board in a MicroATX package. The higher price tag is justified, I think, and I'm happy to spend a little more on what's ultimately the better motherboard. Besides, if I were on a really tight budget, I'd be eying Gigabyte's GA-P55M-UD2. The UD2 is similar to the GD45 but runs about $15 cheaper, and you get two digital audio outputs.