What's better: dual-channel memory mode or a bit more RAM?

[compdoc]

> I'll try the memtest86+.  How long does it take --- roughly, so I can
>plan it in --- minutes, hours, all day?

If you think the ram is fine, you only need to run memtest long enough to
read how fast the memory is. It's listed in Megs per second. It also shows
you how fast the cpu cache memory is.

If you want to actually test the ram, let it run several tests - or
overnight if possible.