There are a couple things I should mention here. The V5 will not fit in all cases, because it is so long. There are other issues as well: On the Asus A7V, one of the most popular Athlon boards, there is a problem plugging in the IDE cable with a V5 5500. The card is so long you have to cut the plastic top off the IDE ribbon to make it fit.
Also, the Geforce 2 GTS is much faster than the MX. I don't imagine even the GTS costing more than the V5.
Last, but definately not least, 3DFx was purchased by Nvidia. This means that driver updates will likely not be available for long. If you intend on keeping the card for long this may become a problem. In addtion the V5 is super power hungry, you better have a 300W power supply. (Same thing goes for the GeForce.)
As for which card is better, it's been said many times. The GeForce will be faster (even the MX in many cases), the V5 will look better. There is not really any downside to either card, but I will say getting a GeForce card happy in a VIA chipset board (which 90% of current Athlon Thunderbirds are), can be kind of tricky.
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bloom25
THUNDERBIRDS