Agreed on the SSD drive for Windows & most used/performance critical stuff.
My friend PC use a old PATA 160G HDD (and not even the black WD performance edition, but blue mediocre one) and just stepping down from 500G PATA WD Black HDD feels like the computer is draging a lead brick or two around. It just suxx. So ancient HDDs are first to dump.
The rest depend on your budget and goals. GTX 660 is not that bad card, after all. With reasonable settings (no high or ultra, no no) it can go well in almost every game. And actually it is a well balanced to the used CPU. If you add much faster graphic (think 780ti or 290X), then you will be CPU limited in most, if not all, cases.
Only one thing you can benefit in terms of GFX card - use AMD one. On slower PCs, the Mantle give as much, as almost 50% faster framerates. So a AMD card will help you big time, even with current CPU, if the game you play most utilize (or at least plan to) Mantle. Like BF4…
If not, then you did not benefit much from GFX upgrade anyway, so it will be doubtfull, if is not better to start saving for completely new rig, if you need more power/fps in games.