So, should I avoid buying MB with sideport at all? As I understand it can give me (i'm not a gamer) only driver problems?
I don't see sideport memory causing driver problems, I just don't happen to know off the top of my head if the Catalyst Linux driver makes use of it today. I think it does, just not sure.
IGPs give you a nice inexpensive graphics solution. You won't get as much performance as a discrete graphics card but you pay a lot less as well. Sideport memory gives you another option - a bit more performance for a bit more money, but still less expensive than the cheapest discrete graphics card.
In terms of whether to get a board with sideport, I think the question is whether you are likely to upgrade your system by adding a discrete card in the future. If you are, then I wouldn't bother with sideport and save the few $$ for your upgrade GPU. If you are likely to stay with the graphics on the IGP, then sideport memory can give better performance and at least slightly reduce the load on system memory.
It is a tiny bit slower, but not that extreme with ddr2/3 in dual channel. There the base speed is high enough. Some nv onboard only handle single channel that will decrease speed a bit. But usally memory thruput is high enough for the cpu that you will not notice the difference. You do not save minutes when you add a gfx card when you compile a kernel or so. You can compare that like when you would use cheap standard memory and somebody else uses oc memory. You only see the diff in memory benchmarks.
I have an Asus m3n78-t with the 790gx and I only use the sideport memory and it runs great. I did notice a lot of corruption when I was using both sideport and ram, until I noticed the speeds the memories were running at -- I overclocked my ram to run at the same speed as the sideport, and voila, no more corruption.
I like my ram so I only run on sideport but its fast enough to run everything smoothly without too much harm on the CPU. When running with PCI-E 7950-GX2 with videos load, cpu is at ~20-30%.
Same thing with the integrated AMD 3300HD cpu is ~30-40%, so its not top of the line, nor does it have a gig of vram, but it works.
Its a great board, don't know about the 4200 but there are FGLRX drivers so it will probably work.