STYLETHREAD -- LET'S TALK SHOP!

I'm seeing my not-quite-over-him ex in a couple days, and I hate to say that I'm thinking like this, because we are still friends, but I do want to look extra-good when I see him. Can anyone suggest a top (that I could wear with jeans or a jean skirt) that is sexy but still casual enough that it doesn't look like I'm trying too hard? I need new tops anyway, because I've gained a bit of weight since last summer and it has all gone straight to my boobs - which means that the shirt can't be too low-cut because I wear a DD and I don't want to be hanging out all over the place! I guess it would help if it's at a chain store, because I won't have time to order something online, I'm seeing him in a day or two. TIA!