There's an article here on Tom's Hardware Guide (I don't have the link handy) which talks about exactly that subject. Have a look and you should find it.
There's a catch - if you put SSDs into RAID, you lose TRIM support. Buying a single larger drive might be smarter.
Also, bear in mind that if you lose one drive on a RAID 0 pair, you've lost all the data (because you effectively have half of every file on each drive). With two drives, you are more likely to have one fail than if you have one drive.
If you really need extreme screaming off-the-wall speed, you might consider one of the PCI SSDs - they are very fast indeed. There's a review of them on Tom's, too.
What is the best mid-end SSD for RAID 0? I thought about Crucial M4, but I read that they have bad GC, which is important in RAID.
Marvel Based Sata III drives are slower than the SF22xx based SATA III drives, BUT are more probmatic. CG is not as good on the M4 as some of the other Marvel based SSD, I think the Intel 510 is better and possibly the Plextor M2 (But Plextor m2 is rather low on random 4K read writes.
Now on raid:
I've used raid almost exclussively since mid 90's - Not one problem. Stopped using when I started buying SSds as I see no benifit. As Compusivebuilder indicated; the Possibility of a drive failing thus lossing all data is higher - It's just that, a possibility and as all disk drives should be backed up, I would not be concerned. (Note the problem is not that half a file is on one disk as 3/4 of all files on a boot/program drive are on one or the other drive - strip size is typically 128 K.
To use raid0, or not is highly dependent on what you are using the drive for, If boot + programs then NO advantage. If the raid0 drive is to be used for working with LARGE video files, large spreedsheets, CAD/CAM drawings, and/or spend a lot of time editing large jpeg Photos, then Yes Raid0 makes since. These are large files that take advantage of fast sequencial read/writes.
Remember Raid0 significantly improve Sequencial read and write (Least important for a boot/program drive, Most important for LARGE file manipulation. Raid0 does NOT improve access time, and very little to no improvement in random small file read/writes.
Does SSD in RAID0 give a boost in performance compared to single SSD (bigger capacity)? Is it worth to put 2 SSDs in RAID0, because price is the same? I read that bigger capacity SSDs are faster.
Depends. I think 3 SSD drives make the most sense. Use one single drive as a system drive - it will have TRIM support and be much faster than a mechanical disk. Use the other 2 in a RAID-0 as a high-performance application drive. They won't have TRIM support but if you notice a slowdown over time you can easily copy the contents to another volume, then perform a secure-erase and then copy the data back. That will solve any problems you have from lack of TRIM. Where SSDs really slow down (in WRITE speed) is when they are constantly full or near full. So try to use your SSDs only for high-demanding applications where you need the speed. Have a separate volume (like a RAID-1 of mechanical disks) for your less-demanding data (like pictures and movies, etc). Remember, SSD slow-down over time is mostly just slowdown in WRITE performance. But either way, SSD read/write performance scales incredibly in RAID-0 and is not worth dismissing for the demanding user.