I'm no expert, but I would do the card, then the monitor and lastly Windows.
The monitor you have looks good enough that the new video card would give more of a difference than the new monitor, unless your current one has some display issue.
Unless you're willing to get Windows 8 (as the upgrade to 8.1 is free) I would wait that one out until more issues are fixed. Windows 8 never gave me problems, while on 8.1 my PC won't go to sleep automatically, in manual sleep it randomly wakes, has been restored twice due to a constant restart loop and a couple BSODs. It's disappointing, because 8.1 does have some nice changes, but it's just too unstable. Maybe by the time you get around to picking 8 up, 8.1 will be fixed.