i want to know which is better for me 5850 CF or 5870 or 5970 for my 23" lcd.and i also want to know that overclocking graphic card is safe?if yes then how much time we can play on overclocked gpu (because of heat).and does this void warranty.
Start with a single 5850.
If you need extra performance down the line, add a second 5850 then.
The 5870 will give you about 20% higher performance but costs tons more.
A 5970 is only slightly faster than a pair of 5850's and is priced a bit higher.
A single 5850 will give you quite good performance at 1920x1200 (or 1080p, which ever your monitor does) and is the most cost effective choice for you.
5970 requires a full tower case. CF 5850 will give you similar performance to the 5970, but requires slightly more power. CF 5870 gives you more performance but uses up a lot more power than CF 5850 or 5970.
Yes, overclocking is generally safe.
Just stick to using the CCC overclocking utility and you will have no problems.
Worst case, you will push the overclock to high and have the computer lock up while gaming (with no damage to the GPU).
IF you bump the voltage (and have no clue of the chips limits) you do run some risks.
Most of the time you can still get quite good, and 99% safe, overclocks without touching your cards voltage.
Even programs that do let you change the voltage usually keep it within safe limits (unless you have an ASUS card/BIOS ).
What do you need that power for if you are using a single 23" display? 23's are almost always 1920x1080, and at that res, a 5850 or 5870 will play just about anything at the highest settings. I would suggest just getting one to start with and see how you like it. If its not enough, and you feel like you need more, THEN get a second.