I'm going to go for an unusual choice, and recommend flat out that you stick with your exisiting card if 2-D is what matters to you most. I ran that card myself for 2 years, and I still have fond memories of it for 2-D work. I also have experience with several ATI and Nvidia products, and there are advantages and drawbacks to all of them.
Obviously, for gaming the G400 is pretty bad (I know from experience that the Geforce2MX beats it easily!). In your $50 price range, you would have to go for an entry-level GeForce3 or Radeon 7500 (or maybe a 8500LE if you are lucky). The Geforce3 would probably be the best gaming choice, but the worst for 2-D work.
Although this is very subjective, most people (and my own experience) tells me that generally ATI cards are better for sharper 2-D images, whereas Nvidia cards give a richer color saturation. Richer colors are not necesarily a good thing - most people like it to play games, but I find it makes it harder when doing accurate 2-D work in photoshop. My personal experience with Matrox cards (both the G200 and G400) is that they are superior to both ATI and Nvidia for 2-D work, as they have the best mix of sharpness and color saturation.
Whichever way you upgrade, I think you will find your previous photoshop work will look a little "washed-out", since your Matrox card has a darker adjustment of midtones that the other two cards (i.e. images seem "blacker" on the Matrox cards). This will probably be more noticeable if you switch to an ATI card. If you switch to an Nvidia card, you a likely to find a bit better color match, but at the cost of being a bit "fuzzy", especially at higher resolutions. Again, that's just a general observation over several cards in each product line.
Probably you best choice overall for an upgrade would be the ATI Radeon 8500, as it gives a good balance of quality and gaming performance. But you need to realize that nothing will look like the Matrox.