In short the prefix for Nvidia cards is akin to the generation of the chipset on the card itself.
So 8000, 9000, etc, progress upwards. However, The only 2 to 3 cards within that range that are really gaming cards are the 600 or 800/900 subsets.
Usually when a new card comes out it's an 800. That's the standard performance card. Then they often produce a 600, often these are slower clock speeds, weaker chips that still run just at lower performance (still good for gaming most times). Sometimes (not always) they come out with a 900 series, which is like an overclocked or somehow improved card over the standard. The other numbers you see (9100, 9200, 9300, etc) are budget, stripped down, cheap-as-possible cards not suitable for gaming. Usually the lower the second number the worser off the performance is. Many of these cards require system memory and have no performance (so slow they don't even require a cooling fan!).
The last 2 digits are for subsets. So you might see a GeForce 6500, and you might see a GeForce 6550 -- these are dependent on the cards rather than the chipsets, what features are enabled on the card, I think.
That's a general rundown for the numbers, as they USED to be. The new cards started an entirely new numbering system that I don't fully understand yet, but I think this is what they did. The top-end gaming cards now are the GeForce GTX 280 cards. These include GTX 260, 280, 285, and 295. I believe they skipped the 1 prefix (for whatever reasons, to avoid confusion with older cards maybe?) and removed one of the trailing suffixes.
So we're on the next geneation chip (first digit 2, next new card they create may start with GTX 380, for example), and their premiere chip is the 280. They've since added the lower but cheaper 260 (still a gaming card), and also 2 improved cards (285/295). I think the pattern will repeat from now on just starting 380, 480, 580, etc. When you see the budget cards with these GPUs they'll probably be 281 through 285.
ATI Radeon cards are a bit different. I'm not fully versed, but just from the cards I've checked over, there's 2 main lines running right now. The "X" cards ("X1600" for example) are usually cheaper, lower end, but still able to game with. Compare them to GeForce 7600 I guess. The X1600 and X1800 are gaming-capable cards, but the others, like X1350, for example, follow NVidia's pattern of budget-card-bare-bones.
Newer ATI cards are the HD line. HD for Hi-Def, from what I can tell. They're meant to have HDMI outputs, I think (most of them seem to have it). They started with the HD 2600 series. Avoid the HD 2000 line. Runs super hot and not the best. The next in line is the HD 3850/3870. The 70 suffix is the higher clocked, higher-performing card, while the 50 suffix is slightly slower but much cheaper. These cards gave Nvidia a run for their money for a bit, and are still very new and quite capable. Just recently the latest is the HD 4850/4870 line (same pattern for the 50/70 suffix).
Unlike the other numbering systems, it seems anything below 3850 on the HD3000 line, for example, is worse and worse the lower the number goes. 3650s, for example, having significantly slower performance than the 3850s (and not much steps between the two, that I've seen).
The GeForce GTX 280 and the RADEON HD 4870 are the most recent and highest powered cards for their respective lines so far.
One other thing to note is that most new cards (Ge9800 and up, and HD3850 and up) require PCI Express 2.0 (16x) -- this is a new format of PCIe port that is only on newer motherboards. If you have one of these cards and put it into a standard PCIe x16, you won't get full potential out of them. They may bottleneck, or simply never run at full input/output because of PCI bus limitations.
That, in a nutshell, sums up the recent cards at least. It should cover anything you want to buy nowadays.
