I have SGA GT5830i and 250MB RAM.
I want to increase its RAM.
You can only artificially increase its RAM by using SD card.But this would go for short time only as the read/write process would worn out the life of SD card so that it won't be usable for you in long term.
Similar type of question was discussed previously in the link below -
Can memory from a memory card be used to increase RAM on an Android phone?
There is a way to virtually increase the RAM, though rooting your device and using a micro SD would be needed. It's best not to really try as the performance may not be as good as you wish, and there could be many issues that occur from doing so. Increasing storage is simple if there is a micro SD slot, but RAM is a different story for phones and tablets. For more information, go here: http://forum.xda-developers.com/showthread.php?t=2099787 and http://techcure.wordpress.com/2012/08/31/increase-available-ram-in-android-using-swap/ Original thread here: http://forum.xda-developers.com/showthread.php?t=1659231 It really doesn't increase RAM, but uses swap instead.
Use the phone's sd card storage as RAM.
You're phone has to be rooted though and the sd card needs to be a fast one for you to see any noticeable difference.