Upgrade Intel iMac's RAM from 3GB to 4GB and change Graphics Card

They almost certainly will be willing and able to do that, but you'll have to get an estimate from the store on the cost. To get a ballpark figure, check Apple's website and see what 4GB of DDR2 SO-DIMMs would run you, then add about $100 for labor. Honestly, there's no real point to paying someone to do this. All you do is take a Philips #2 screwdriver to the screw on the little panel at the bottom of the iMac. Once you losen that, the panel comes off, and there are little black plastic flaps that let you pull the RAM out so you can grasp it with your hands. Then you just push the new RAM in until you feel it snap into place, fold the black plastic tabs over it, and put the panel back on. Once you have the RAM, it'll probably be a 5-10 minute job, tops. Just be sure not to do it while the system is on or plugged in. If you do it yourself you can buy DDR2 SO-DIMMs anyone. Apple will charge a hefty premium because they have to cover the costs of potentially having to replace them under warranty.

As for the video card... It's technically possible, but it would be so ridiculously expensive for just swapping the equivalent nVidia card for your ATI one, it would be downright stupid to pay to have it done. So for all practical purposes, it's not possible.