In your range... HD5750 is the only one that fits. End of story. There are some other lower version cards from nVidia and ATI, but they are not worth investing in right now.
And yeah, you can wait for GT300 but the thing is i don't think it would help much coz nVidia cards are always overpriced!
We don't know, how they are gonna priced, so don't take it.
The reason for the wait is to see
if NVIDIA cards are placed right.
we will definetly see price drop in ATI cards,
which makes the whole thing worth waiting