how are you recommending a processor without knowing the specs to the prebuilt? The specifications list
shows that the AIO comes in multiple SKU's meaning multiple different configurations. To add, I don't think throwing money at a near two decade old system will bring about any positive returns on investments.
Regardless, might want to parse the specs to your AIO or an SKU to it. As for the build, how much can you spend if we/you were to look at a new build to get your feet wet?