Well I would say that you need to wear something that you are completely comfortable and confident in. Its better to be wearing something not as flashy that you feel good in, than something too flashy and makes you a little self conscious.
I would go something a little more flashy, something that shows off your best features and will make you stand out a little from the crowd. The most important thing though .. is that YOUR the one that feels uber HOT in your outfit.
Its your confidence that is going to win him over. What your wearing will merely get his attention.
I may not be the norm here, but if she wants me to notice here, skirt and heels all the way. If the weather permits. I am a standard nice business style guy, but a girl I would be interested in couldn't dress to slutty for me.
That said, I have never interpreted dressing very sexy as the intention of having sex. I think she is merely a confident women that looks good and is proud to show it. She'll have my eye.
Dont wear something too flashy, and don't wear pastels.. that are too dim-ish. Wear something flirty, like a mini-skirt... and if you are not into skirts, try on tight shorts or jeans. It also depends on what kind of party is it. If people will be wearing dresses there, wear a black or red dress. But if this party is one organized at a house, then wear something casual... like the mini-skirt and a tank-top that is colorful