nothing is bottlenecking on that system unless something is damaged...
the hdd is slow but not exactly system breaking... i dont have an ssd and my games run just fine. i do have load speeds of 200Mbps but theres not a hugley noticeable difference coming from 100Mb which is where your drive fits...
you would benefit from getting another identical drive and raid 0'ing it but there would be little you would see in games other than lower texture pop.. so no reason to get an ssd just yet...
chances are its a driver issue... or the actual games your playing. if they dont stress the gpu then you may get an over abundance of fps, the problem is when you run in to an area that suddenly increases the stress and dropps the fps you will really notice the droop...
its better to limit your fps to the rock solid minimum in games which eliminates that droop all together...
bf3 for instance your system will run it at high but will do that at 45-60 fps with an absolute max of over 120. but if you reduce the gfx settings so you get a rock solid 60 minimum and then limit the top end fps to 60 the game becomes ultra smooth with no stutter lag or anything like that...
also some games prefer hyper threading to be disabled, again helps reduce stutter.
reason ? if hyper-threading is enabled the cpu will use 2 threads on 1 cpu b4 it uses the extra cpu core and as you know hyper-threading is a little less efficient than an actual cpu core. this slight drop in efficiency can impact a game
dramatically...
bf3 is a game that does not like hyper threading when 4 cores are available it just runs smoother when its disabled.
online the fps issues become more of an issue you may want max settings but that wont help you as you need good consistent packet data flow and without it you will get laggy gameplay... again its better to reduce the settings to get 60 fps minimum. because thats whats genrally accepted as playable for fps games... the chances are atm your getting owned by guys running at minimum. but look good while doing it... personally id rather take the gfx hit and own them...
if you want 60 fps on bf3 @max settings then your looking at a gtx 670 minimum.
a 570 will not cut it im afraid...