I'm not even sure if the .NET framework provides a class to access a process's memory. so what you're probably going to have to do is make a call into unmanaged code(win32).
i believe there is a class called "memory" under process class in win32.
what i would do is get the process id of the process you are trying to read/write the memory of and then make a call into this unmanaged win32 to perform the reads/writes.
if you're just looking to hack something, you're rebuilding the wheel(
http://www.artmoney.ru/) this app called artmoney is the best app i've ever come across for finding values/ranges in a process's memory and changing/freezing the values.
if you're just doing it for fun then by all means give it a stab and post back your results.