This will happen! In fact, it's guaranteed to happen, because both stock levels and prices change quickly. So how do you know if that graphics card you've got your eye on is a good buy in its price range?
Here are two resources to help you judge if a graphics card is a good buy. The first is the graphics card hierarchy chart, which groups graphics cards with similar overall performance into "tiers." The top tier contains the highest performing cards available, and performance decreases as you go down the tiers from there.
You can use this hierarchy to compare the pricing between two cards, to see which one is a better deal, and also to determine if an upgrade is worthwhile. I don't recommend upgrading your graphics card unless the replacement card is at least three tiers higher. Otherwise, the upgrade is somewhat parallel, and you may not notice a worthwhile difference in performance.
At the request of readers, I have added mobile and integrated chipsets to the hierarchy chart. I want to make it clear that there is very little performance data available for these graphic solutions. While the discrete video chipsets on the chart are placed based on a lot of information, many of the laptop chipset positions in the chart are guesstimates based on their specifications. At worst I don't think they're more than a tier away from their actual performance, but this is something to keep in mind when considering mobile graphic chipsets.
|8800 GTX, 8800 Ultra||3870 X2|
|8800 GT 512 MB, 8800 GTS 512 MB|
|8800 GTS 640 MB, 9600 GT||HD 2900 XT, 3870|
|8800 GS||3850 512 MB|
|8800 GT 256 MB, 8800 GTS 320 MB||HD 2900 PRO, 3850 256 MB|
|7950 GX2||X1950 XTX|
|7800 GTX 512, 7900 GTO, 7900 GTX||X1900 XT, X1950 XT, X1900 XTX|
|7800 GTX, 7900 GT, 7950 GT||X1800 XT, X1900 AIW, X1900 GT, X1950 PRO, HD 2900 GT|
|7800 GT, 7900 GS, Go 7950 GTX, 8600 GTS||X1800 XL, X1950 GT, Mobility X1800 XT|
|6800 Ultra, 7600 GT, 7800 GS, Go 7800 GTX, Go 7900 GTX, 8600 GT||X800 XT (& PE), X850 XT (& PE), X1650 XT, X1800 GTO, Mobility X1900, HD 2600 XT, 3650 (DDR3), 3670|
|6800 GT, 6800 GS (PCI-E), Go 7800, Go 7900 GS, 8700M GT||X800 XL, X800 GTO2/GTO16, Mobility X800 XT, HD 2600 PRO, Mobility HD 2600 XT, 3650 (DDR2)|
|6800 GS (AGP), Go 6800 Ultra, Go 7600 GT, 8600M GT||X800 GTO 256mb, X800 PRO, X850 PRO, X1650 GT, Mobility HD 2600|
|6800, Go 6800, 7300 GT GDDR3, 7600 GS, Go 7700, 8600M GS||X800, X800 GTO 128mb, X1600 XT, X1650 PRO, Mobility X1800|
|6600 GT, 6800LE, 6800 XT, 7300 GT DDR2, Go 7600 (128 bit), 8500 GT||9800 XT, X700 PRO, X800 GT, X800 SE, Mobility X800, X1300 XT, X1600 PRO, HD 2400 XT|
|FX 5900, FX 5900 Ultra, FX 5950 Ultra, 6600 (128 bit), Go 6800 (128 bit)||9700, 9700 PRO, 9800, 9800 PRO, X700, X1300 PRO, Mobility X1450, X1550, Mobility X1600, Mobility X1700, HD 2400 PRO, Mobility HD 2400 XT, Mobility X2500|
|FX 5800 Ultra, FX 5900 XT, Go 6600, Go 7600 (64 bit), Go 8600M GS||9500 PRO, 9600 XT, Mobility 9800, 9800 PRO (128 bit), X600 XT, Mobility X700, X1050 (128 bit), Mobility X1350, Mobility X1400, Mobility X2300, Mobility HD 2400|
|4 Ti 4600, 4 Ti 4800, FX 5700 Ultra, 6200, 8400 GS||9600 PRO, Mobility 9700 (128 bit), 9800 LE, X600 PRO, Mobility X600, Mobility X1300, Xpress 1250, Mobility HD 2300|
|4 Ti4200, 4 Ti4400, 4 Ti4800 SE, FX 5600 Ultra, FX 5700, 6600 (64 bit), 7300 GS, 8400M GS||9500, 9550, 9600, Mobility 9600, X300, X1050 (64 bit)|
|3 Ti500, FX 5200 Ultra, FX 5600, FX 5700 LE, Go 5700, 6200 TC, 6600 LE, 7200 GS, 7300 LE||8500, 9100, 9000 PRO, 9600 LE, Mobility 9700 (64 bit), X300 SE, X1150|
|3, 3 Ti200, FX 5200 (128 bit), FX 5500, Go 5600, Go 6200, Go 6400, Go 7200, Go 7300, Go 7400 (64 bit)||9000, 9200, 9250, Mobility 9600 (64 bit), Mobility X300|
|FX 5200 (64 bit), 6100, 6150, Go 7200, Go 7400 (32 bit)||9200 SE, Xpress 200M, Xpress 1000, Xpress 1150|
|2 GTS, 4 MX 440, 2 Ultra, 2 Ti, 2 Ti 200||7500|
|256, 2 MX 200, 4 MX 420, 2 MX 400||SDR, LE, DDR, 7000, 7200|
|Nvidia TNT||ATI Rage 128|
There you have it folks: the best graphics cards for the money this month. Now all that's left is to find and purchase them, and we leave that part up to you. The best prices will almost certainly be found online, but sometimes large retail outlets might surprise you with a good sale. (Compare Prices on Top Video Cards)
Don't worry too much about which brand you choose, because all of the cards out there stick pretty close to the reference designs by Nvidia and ATI. Just pay attention to price, warranty and the manufacturer's reputation for honoring that warranty if something goes wrong.
Also remember that the stores don't follow this list. Things will change over the course of the month, and you'll probably have to adapt your buying strategy to deal with fluctuating prices. Good luck!