Easy as to what I would do, if I achieved the goal that we wouldn't know until the end of the 2 month period. Firstly I would never set a financial goal unless I knew and had proven myself to be able to beat the game at a certain winrate. That way you know based on volume if something is possible to achieve or not.
So if you had a 5bb/100 winrate of 5nl that is 1$/400
hands to at that stake only you would have to play out 320,000 hands to reach your goal (but obv your going to move up stakes as your br increases. But lets say you don't, then your talking 10k+ hands /day to do this in a month or 5k+ in two months.
Anyhow math is math, I'm sure you can get the gist from above. Basically I would apply a 40-50bi rule and grind cash at 5nl to start and moving up as the br allowed. The primary thing we can control when playing poker is the volume we put in, especially talking online when you can run loads of tables (if you can do it profitably), so if your a winning player and put in the volume any financial goal can be achieved eventually it's just a matter of how long it is going to take.
Thus to say i'm going to make 800$ in two months is an unknown, whereas saying I'm going to put in 320k hands of 5nl in two months is within grasp if you can put in the time. Just my thoughts essentially on the cash side of things.
On the SNG side well its similar, we have primary control over the volume and ROI is the key for financial gains. To stick with a similar example of 5$ sng's (although only 40bi to grind sngs I'd consider rather agressive, probably prefer at least 75-100 myself) then you have to know an ROI (return on investment), say you have a 20% return on investment so for each sng you play you profit 1$ on average then you will have to play 800 sng's at the limit to hit your profit, so your talking 15/day to hit the goal in 2 months (grinding however 15 sng's / day is a small amount unless your doing it 1 or 2 at a time i guess).
Now to sum up its all about volume and your winrate, bb/100 in cash ROI in sng's/mtt's. I know to have a fairly good estimate on your cash winrate for a limit you are looking to want a sample of 50-100k hands for long run. For SNG's etc however I am not sure what a good sample would be to determine your average ROI that should hold through for the long-run, I am sure we have a couple of sng grinders that could comment on that however i wouldn't be surprised if its 500-1k sng's maybe.
These are my toughts, as mentioned to answer your question specifically I'd grind cash games because this is currently where I am looking to focus and improve my game.