Question regarding performance

[LegatusDivinae]

So, I have

i5 750 at 2.4 GHz
Radeon 5850
4 gb ram
650 W Corsair PSU

1.What kind of FPS should I be getting with:all settings that slow CPU to 0 (flora=Off, flora=0, shadows=0, light. shadows=0, particles=low), most of graphical settings at medium except graphics quality which is high
2.My processor idles at around 60 degrees C. I know that's too high, I am planning on installing new cooler and replacing thermal paste. How major of performance upgrade will this be, other than obvious saving of my PC from burning?
3.I plan on getting 1x4Gb RAM more (DDR3 also), how much of performance improvement do you think it would be?
4.I heard my processor is good at OC'ing. I won't be OC'ing it to 4.2 GHz, altough I've heard it can be, just to around 3.2-3.5 GHz (WHEN I get the stock cooling and learn more regarding the topic). How much average FPS do you think I would gain there?
5.Are there any other obvious minor improvements that are software related, or are hardware related but CHEAP (like not changing your CPU or GPU) I could apply to improve my FPS?

[Rbstr]

I don't know what specific FPS you're going to get.
It'll be playable. Try it an find out. It's free.

60C is too high to idle...are you on the stock fan? Dust it out at least.
A good air cooler will knock that down a lot.

More RAM won't help unless you've got enough stuff in the background so PS2 can't take up it's 2gb maximum.

[Ailos]

The memory upgrade will probably be your biggest improvement IF your modules are symmetrical. If you've got 2x2 GB in there now, DO NOT fill in just a single 4 GB module, it HAS to be another pair of 2x2. Also, don't mix memory speeds (DDR2 won't work with DDR3 and new fast modules will slow down to your old ones).

As is, I would expect ~25 FPS on medium settings, but expect a lot of hiccups in heavy fights, where your average FPS is still high, but you get occasional spikes of lag. A good RAM upgrade and proper overclock will mostly address these hiccups, won't do much to your top framerate.