The B85M-ITX does indeed support the G3258 and is able to OC it. However, you will have to make sure you have the BIOS version of 2.10 or newer as the support was added in that firmware. You can find the correct version here: http://www.asrock.com/mb/Intel/B85M-ITX/?cat=Download&os=BIOS
It's hard to say if the switch to G3258 is a good one. The i3 has hyper threading, so it basically has four logical cores to run things on, whereas the G3258 only has two cores (no HT). This will make the i3 faster in multi-threaded applications, including many games, productivity software and general multitasking. Some games don't run on two cores at all (for example Far Cry 4). On the other hand, when you OC the G3258 to above the i3's stock frequency (3.50Ghz), it will perform far better in single and double threaded applications (such as Dolphin) as it is not hard to get it running at say 4.20 - 4.50Ghz.
Your choice. I'd recommend checking how many multi-threaded (4+) programs you use, and if you don't have any (or at least many) then consider switching to the Pentium if you have the money.
It's hard to say if the switch to G3258 is a good one. The i3 has hyper threading, so it basically has four logical cores to run things on, whereas the G3258 only has two cores (no HT). This will make the i3 faster in multi-threaded applications, including many games, productivity software and general multitasking. Some games don't run on two cores at all (for example Far Cry 4). On the other hand, when you OC the G3258 to above the i3's stock frequency (3.50Ghz), it will perform far better in single and double threaded applications (such as Dolphin) as it is not hard to get it running at say 4.20 - 4.50Ghz.
Your choice. I'd recommend checking how many multi-threaded (4+) programs you use, and if you don't have any (or at least many) then consider switching to the Pentium if you have the money.