what would be the best video card replacment?

The GTX 720 is about the lowest power PCIe card out there, if your computer has a PCIe slot.

Must be an old game you want to play if it needs a graphics device that uses shader model 3. Modern cards use shader model 5 with full support of previous shader models. The 3100 is an integrated graphics device...a chip that is part of your systems motherboard and not a "card".

With no details of your system, we have no idea what card slots are available on your motherboard for a card, nor how many watts your power supply outputs to determine what card can be supported, nor if your systems case is a standard ATX which can use a full size card or if it's a slim system that can only use a low profile card.

Useful info-
Prebuilt system....make and model number (Acer Aspire xxxx, Dell Inspiron xxx, HP Pavilion xxxxx, etc.)
Custom system....make and model of motherboard and same for power supply