"It's the thought that counts" may be a nice sentiment, but it's certainly not the reaction you want her to have when she lays eyes on the ring.
Before selecting the ring, consider your girlfriend's personal style and taste. Does she favor contemporary or antique jewelry? Would a yellow-gold band match her jewelry collection, or does she tend to sport platinum or white gold? Politically minded brides will want to avoid conflict diamonds, so bear that in mind when selecting a stone.
It might be wise to enlist the help of her mother, sister or best friend, provided she can keep a secret. If you don't mind dialing down the surprise factor, you can even ask your girlfriend to design the ring of her dreams online or take her to a jeweler to browse. (While you're there, be sure to have her ring finger sized!)
Trust us -- even if she picked the ring herself, she'll still be thrilled during the proposal. And particularly picky brides will probably thank you for letting them participate in the process.