Hi, I think you should try samsung 950 pro nvme ssd drive mounted on a PCIE x4 to m.2 x4, it's one of the only nvme ssd that allow legacy boot, indeed they have legacy oprom. This should be the fastest bootable drive you can get for legacy motherboard, it should work as far as pcie goes, I tested it on P35 board and it was show as a regular drive, but you can't use UEFI/GPT partiition scheme on it otherwise it would not work on this platform. I think that you should be able to boot regular NVME drive that don't have legacy oprom by using a bootloader like clover on an USB stick then booting on the drive.
Also when overclocking this skulltrail platform you should try to push as far as you can the FSB, indeed the FSB is used between the CPU and the NB to communicate and it is also used between the NB and the RAM, so higher FSB means better performance (because sometime it is very limited by the ram bandwidth), so don't be lazy by only pushing core multiplier
When you will upgrade the ram you should buy 32gb of FBDIMM 4RX4 PC2-6400F, all of the 32Gb should be usable, if it's only 2RX4, only 24gb should be usable : https://www.overclock.net/forum/27566598-post353.html
I'm not sure (never tried before) but if you want USB 3.0, M.2 nvme SSD and 10Gbits lan you can combine USB 3.0 and M.2 nvme SSD on the same riser, there is pcie riser that allow the use of multiple M.2 nvme SSD, so if you grab a 950 pro along with an USB 3 adapter like this one : https://www.alibaba.com/product-detail/NGFF-M-2-Key-B-M_60795350477.html it should work (not sure at all, you should test it in a cheap testbench)
Or if you don't want to use M.2 nvme SSD you can use combo card that pack USB 3.0 controller and SATA 6gbit/s controller like the ASUS U3S6
For this board, pcie x16 slot are 2.0 and come from the MCH (also called NB)
edit : and for the next board, you should test the famous dual 1366 EVGA SR-2 board