Set it back up, then go down to the shop and remind them of what they told you. Tell them that the guy told you if it didn't work they'd look at it, and don't let them charge you for it! They sold you the card, when you asked them if it would work on your system. So just to have them look at it and see if everything's plugged in right shouldn't cost you a dime. Now, if there's some work that needs done, that's another thing. Whatever you do, though, don't take an antagonistic stance with them. Just explain to them what it sounded like you were promised beforehand, so everyone's on the same page and you don't get unexpected charges.
However, given your system specs, I'm pretty sure that guy had no business selling you that card. I've done pc sales before, and I'd have told you to upgrade your processor first. The K6 series is fricking Pentium I era! Why he'd sell you a fairly modern video card for that system is beyond me. You're there, and I'm here, so bear in mind this (and my last statement) is just my guess, but I'm gonna guess you're using pc100 memory at the fastest. Certainly not DDR. Raptor is absolutely right about the minimum system requirements; I personally wouldn't try to game with anything less than 512 megs of DDR RAM.
My point is that you've got to balance your system. Right now, your CPU/RAM are choke points: limiting factors in your PC's overall performance. In terms of cutting edge gaming, what you've got there is a doorstop with a good video card. If my 600Mhz Athlon isn't cutting the mustard with new games any more (even though it runs stuff that it shouldn't be able to run; I love AMD), then your last-gen, 500Mhz K62 just isn't gonna hack it.
The point of all this is that if you really want to game on that machine you should seriously consider ditching the old box and building a new gaming pc around your nifty new video card.
