64 bit OS under Virtualbox within Win7 pro 32

I actually have a Lenovo ThinkPad X120e, running 7 pro 32 with 4gb.
If I upgrade to 8gb and try to install LMint 64 under Virtualbox, will it deal flawlessly 8gb?

You cannot add more than 4 gig ram to a 32 bit system, You will need a 64 bit system to add more ram.

Welcome to the forum.

In VirtualBox you would assign to the client OS some portion of the hosts accessible RAM to the client. In a laptop I would expect this to be about 3 GB, maybe a bit more. The host will require a substantial portion of this with the remainder (maybe 1.5 GB) available to the client. Do not run the host short. Memory not seen by the host cannot be used. Installing more than 4 GB in the host would be a waste. To have full access to 8 GB RAM you would need a 64 bit host.

You can probably get by if you allocate 2 GB to the VM. But I would suggest installing 32-bit Linux Mint rather than 64-bit Linux mint in the VM.