None. You can have full CHDK support, or HD video. But not both. Not today.
There's only
one PowerShot that meets your video requirement: The
SX1. A few others have lower-res 720p HD video, like SD780, SD960/970, and SX200, perhaps others. The SX1 is the first CMOS-sensor PowerShot, so as far as your image quality concerns go, I'm guessing no other Canon that's not a dSLR will come close. Your price cap, however, is exceeded. It goes for $550-600usd.
Incidentally, it's also the only one that there's progress with CHDK development. They are all too new; The SX1 is beta and supports one firmware, but chances are that if you haven't bought it yet, the one you'd get will have FW 2.01a, which was just dumped a month or so ago. Also the SX200 has been dumped and will probably get some functionality by 2010, and the others, I don't know...
So I guess you need to decide what's most important to you... current CHDK support or HD video? If those cameras are capable of being ported, they will - eventually. But if you're not porting it yourself, you are at the mercy of others. It could feasibly take years, although it might just be six months.
Just to give you one more option, consider a TX1. Sure, it's years old, but you can pick them up CHEAP compared to what they cost before, (
$249 for a refurb @ B&H Photo) And it's well supported in CHDK, and it shoots 720p video.
