I think before I added those 2 x 80mm fans, I was able to get 600 mem speed, and 425 core. But my case is pretty cool since it's winter time; case temp is around 22C-25C all the time, so that also helps me OC.
Also memory overclocking depends on the type of ram chips you have (hynix, samsung, winbond, etc.) Check your ram chips and see who the manufacturer is and go to their site to see the specs on your ram chips.
I had Hynix 5ns chips which come 400mhz stock on 2.5V and can reach 700mhz on 2.8V. Here's the specs on my ram chips: http://www.hynix.com/datasheet/eng/d...DU561622CT.jsp
Since you have the same GPU as mine, the nv44a, I say you should be able to get atleast 450mhz out of the core if you add some kind of cooling to the big heatsink on the 6200.
Anyways, goodluck fellow 6200'er.