What will you be doing with the PC? If you are just browsing the internet (which I hope is the case with a Celeron 2Ghz) you don't really need anything other than something to connect a monitor too. You could really just get whatever is the least expensive.
Is four year old ATI Radeon HD 3850 too good for this system?
I don't know what you mean by "too good." It is certainly better than the rest of the components in your system, so I wouldn't spend more to get it, but if you have one already, laying around, it wouldn't hurt to use it