I have a Dell OPtiplex 755.
the graphics its processor supports is 64 mb that can increase up to 256mb. i want to record my screen. but the pc lags in screen recording. can i increase the graphics enough so that i can record my pc screen without any stutter?
There are 2 types of encoding (recording on PC) hardware which uses the video card and software which relies on the CPU. Given the age of your PC either will be a bit of a struggle, from what you mention about your video it's sounds like it is onboard graphics which isn't going to work. Using the CPU may be a possibility depending on what else you are doing on the PC that you want to record and how low of a quality you are willing to go for the recording.
You dont have to buy a new pc if your eager enough to research on a processor that is much better and more powerful than what you currently have that has the same socket as your motherboard, so that you dont have to buy a new motherboard, but if you want you can just buy a new pc if the one that you currently have is too old and if you have the money to do so.