The card will work just fine for you. Don't worry about that distinction.

I believe it has to do with how the card's GPU sends information to the graphics memory. It can use 64-bit strings, rather than 32-bit. It ends up being a similar distinction, but there's no issue. Just like a 64-bit processor can run a 32-bit OS, 64-bit stings can handle 32-bit.

For $32, it's not bad at all. You'll be able to run your Command and Conquer, and be able to take advantage of Aeroglass. Plenty of memory, reasonable clock speed for the price. You'll be fine, probably.

For gaming, though, you might be limited on your resolutions. There's a good chance that you won't be playing above 800 x 600 if you go above low settings, if you want decent framerates. Command and Conquer shouldn't be overly graphics intensive (especially if you're playing an old one), so you might be okay. Expect that you won't be able to use AA or Anisotropic Filtering.

I'm not sure you need to go with a DX10 card, though. A DX9 might do it for you. In that case, take a look at this: Newegg.com - POWERCOLOR X1550 256/64DDR2 Radeon X1550 256MB 64-bit GDDR2 PCI Express x16 Video Card - Retail. $29 with shipping. Higher clock speed, same bit throughput.

If you want to go a little more money: Newegg.com - SAPPHIRE 100195L Radeon X1650 512MB 128-bit GDDR2 PCI Express x16 Video Card - Retail. $40 with shipping. Much better card. Still DX9, but the 1650s are way better than the 1550s and are still considered pretty good for low-end cards. 512MB RAM, 128-bit interface. This'll guarantee you can play with respectable settings. The last one there is also a Sapphire. Brand name matters in cards, and that's one of the best for ATI cards.

If you really want DX10, maybe look at this: Newegg.com - POWERCOLOR 24PRO 256M DDR2 Radeon HD 2400PRO 256MB 64-bit GDDR2 PCI Express x16 HDCP Ready CrossFire Supported Video Card - Retail. $33, but better clock than the 8400GS you mention. I'm no expert on low end cards, but by the numbers, it's worth the extra dollar.

I tend to prefer ATI, especially on the lower end, as they seem to be willing to eek out just a bit more capability and support is better (read drivers). Oddly, you always get a little more bang for your buck (I had a 9550 that in the end had capabilities that it wasn't advertised as having).

If you'd like a bit more guidance on stuff like this, post again. Of the cards, the 1650 is probably your best bet for being happy with what you see in game, but doesn't have DX10. If you want DX10, you'll have to look around and read the reviews of each.

Hope this helps and doesn't cause more confusion.

(Note that the 128-bit cards will inherently be faster than the 64-bit ones, and they will work just fine with your 32-bit OS. People are running 256-bit cards on 32-bit OSes)

Well, here's a decent comparison: DirectX® 10

Mostly, you're looking at some stuff to really enhance high-end gaming. From what I've seen in Bioshock and Crysis, the real difference is in water (if you pump the graphics, it can look amazing). Some will probably disagree. But, the thing is, no low end card is going to be able to cover a lot of that stuff anyway. As with DX9, there will be some residual effects for normal users. But, keep in mind, all of DX9.0c still isn't being taken advantage of.

But, as you suspect, DX10 requires hardware support. A DX9 card can't give the benefits of DX10. A DX10 card can't give the benefits of DX10.1. But, in most cases, you won't lose anything. Few pieces of software have gotten rid of DX9 support. You should be fine for a long time to come. Wait until DX10.1 cards get reasonably priced (there are a few out there from ATI HD 3xxx series, but still cost more than the 1650). You should be fine for quite awhile. And when you upgrade, at least you won't have spent too much on your previous card.

Any time. Please, let us know if you have any problems with your new card when you get it. Or, for that matter, any other type of problem.