NVIDIA GeForce 8400 GS - replace or upgrade?

It ultimately depends on what you want to use a new graphics card for. The Nvidia 8400 GS runs on PCI Express 2.0, however, there is no real world difference from PCI-e 2.0 to PCI-e 3.0 - in games maybe a few frames at most.

If you want to just upgrade your graphics card you can do that as PCI-e 3.0 cards are backward compatible. Additionally, AMD's top of the line motherboards only run at 2.0 as well.. so in terms of overall system upgrade, you can probably just get by with getting a new card.

However, I'm curious about a few things:
1) What would you use your new video card for? (i.e. gaming, video editing, photo editing, etc..)
2) How much do you want to spend?
3) What are your current system specs? (i.e. what CPU do you have, what type of memory do you have, etc..)
4) What is the rating on your Power Supply (i.e. 300W, 400W, 500W, etc..)