I think people don't regard MSI as highly as Gigabyte or ASUS based on past performance. In previous generations their boards often had issues with onboard cooling or inferior OC ability and people remember these issues.
However, with the current generation of sandy bridge board MSI have done very well. They have use high quality components that have the ability to run hot and remain stable (hence reducing cooling requirements), overclock well and include high-end features (like SLI/CF and x8/x8 dual card arrangements) in their lower-end boards - so undercutting their competitors. Hence I would buy a MSI s1155 board with confidence (also, they just upgraded their warranty length to three years).
As for running 1600MHz without OC, as I mentioned - since Intel's spec only rates the s1155 CPUs to run memory at 1333MHz then to run 1600MHz at full speed then you techically need to overclock the memory controller (which is a part of the CPU) no matter what board you pick, it is just part of running higher-speed RAM. This is not the same thing as overclocking the CPU - you can run 1600MHz RAM but also keep the i5 2500K chip at stock 3.3GHz speed.
As for the difference between the C43 and G45, the C43 only has one full-size PCIE x16 slot so it can't support SLI or crossfire. The G45 in contrast has two of these slots and supports both SLI and CF at x8/x8 speeds (ie the best speeds available with a p67 chipset without using a bridging chip) - so the G45 offers you many more options in terms of graphics upgrades for only a few quid more. The G45 also has more USB3 connectors.