Right now the best bang for the buck is a GTX 980Ti. Adding that to your machine in addition to your 450 is likely to give you the biggest improvement, if your Motherboard and Powersupply will handle a second video card. If a GTX 980Ti is outside your budget adding a card that has at least 4GB of dedicated video ram will improve performance. If your motherboard and Powersupply will only handle one card, then replacing your 450 will improve performance.

Retaining the GTS 450 and letting it continue to drive your monitor(s), if possible, will keep your new card doing just rendering and give you the best performance increase.