Samsung 840 EVO 250GB
The Crucial MX100 and OCZ (Toshiba) ARC100 are maybe even better options.
Given the mobo, is there a preference for the best RAM sticks?
I need a modular and quiet PSU hence the Seasonic which seems to be a favourite.
The X-series is a little larger for the case. You might give a look to the Cooler Master V450S and the Corsair CS-450M.
Better units are the Corsair RM450 and the BeQuiet E9 480W CM, but they are as large as the X-series (even if less expensive and less problematic than those contemporary Seasonics).
I plan to put a good gaming GPU in at some point hence the 650W.
With a good gaming GPU you usually won't pass the 300W mark (i.e. with a R9 280X+i5 combo you should be around 270W): with a typical 7970 GHz you should be lower than 350W with synthetic loads. If you overclock your rig, then YMMV.
Anyway, with such a graphics the Seasonic X-series won't fit (as well as the Corsair RM or the BeQuiet E9 CM).
If I dont overclock, can I get away with a fan-less heatsink?
Yes (because you have the largest case fan just nearby the heatsink) but it's pointless: you'll just get worse temps.
What do people recommend?
What SPCR recommends: a Scythe Kotetsu, a BeQuiet Slim, or a slightly inferior Noctua NH-U12S.
By the way, you have not that much room inside the 304 (particularly when you populate the drive bays).