I think you're onto something there! Color quality wise, how many colors is the original image? Your chart looks pretty close to the original to me, but it's hard to tell for sure with all the gridlines in place.
For your other questions - it's highly unlikely that anyone at a store will be able to convert RGB into floss colors for you. Odds are unless you get someone who knows about computers, that they'll look at you like you have 3 heads. There are a number of RGB to DMC converters online. I haven't used them, but here's a link to one:
http://bond.in.th/octrabond/image-tools ... o-dmc-code" target="_blank - if you were really neurotic like me, you could try a couple of different one and see that they both recommend the same color, and if they don't, then jot both down and take a look at the color while in store to decide for sure which you think is the better choice.
Number of stitches per skein of floss also varies greatly. The biggest factor is what size fabric you're using. Cross stitch fabric (aida, evenweave, or linen) comes in different counts, going as low as 11 stitches per inch in aida, and as high as somewhere in the 40 stitches per inch for linen I belive, and then there's silk gauze which goes even higher. So you'll need to decide what fabric to do the piece on. That will also affect the size of your finished piece. For your first peice I'd suggest aida fabric which is designed specifically for cross stitch, with the holes very visible.. and the highest count that goes up to is 18 stitches per inch, with 14, 16, and 18 being fairly common and easy to find counts. The lower the count, the larger the stitches, and also the more strands of floss you'll need for each stitch. To get good coverage on 18 count you need to use 2 strands of floss (it comes with 6 strands and you have to separate them) and if you go to 14 count, you might need 3 strands. So this would have a big impact on how much floss you use.
Here's a link to a chart that works as a very basic quideline though..
http://home.comcast.net/~kathydyer/nf_x ... #floss_amt" target="_blank - It lists the fabric count down the side and the number of strands of floss over the top. I think these numbers are based only on actual stitches, and don't factor in things like the amount needed to start and end your stitching, and and the bits that get wasted because they're too short to reuse. So if your stitch count for any particular color is within 10 - 20% of what it says you'll get out of a skin, I'd go ahead and get an extra one.
Good luck with this! It's a biggie to attempt for your first project, but looks quite managable! And we're all here to help out of you have any questions. And of course please share pictures of your progress!