Good choice on the round stone. I know one thing, defintely look at it in person before you buy it! Take it outside the store if they'll let you and look at it in the sunlight, out from under those purposely placed lights that tend to make even the lesser quality stones look brilliant. I bought my ole lady a 1 carat on a platinum setting with small side stones for an engagement ring back in the summer. Got it from a local jeweler. Go for cut and color over inclusions, depth of the stone really affects light return (sparkle). Too shallow or too deep, it won't sparkely worth a ****, similar to other alternative shaped stones. If you really look at my ole lady's up close, it has a couple tiny inclusions in it, but the cut and color is very brilliant. It doesn't matter what light it's under, it blings like a mo-fo, and unless you are holding it up to your eye, you can't even see said inclusions. You should be able to find a very nice round brilliant cut 1 carat stone for around $3k +/-.
Another tip is if you can find a stone under 1 carat, like 0.93 or something, they will tend to be cheaper than 1 carat and above, and if the depth is right, it looks just as big as a 1 carat for less money. The 1.03 diamond I bought actually came out of a set of ear rings my jeweler had in his vault, then I picked out the platinum setting. Everybody that asks her about her ring asks who helped me pick it out. Haha I done a shitload of research and picked out what I had in mind that looked good. I am not a big fan of all these cluster/halo style rings half my buddies have bought. They have a bunch of smaller diamonds that makes it look like it's big and expensive, when in fact, it's not. I like the solitaire look for sure!
Good luck man!