memory size: depends on what resolution you game at. For example, the two cards I recommend below would be complete overkill and a waste of money if you game at 1024x768. If you do game at 1280x1024 or lower, then you should only need a 256MB card. However, if you game at any higher resolution, you MUST get a card that has 512MB. Otherwise it will be bottlenecked and stutter and fail.
core/memory/shader clock: Not always directly comparable. Two cards with the same clock speeds do not necessarily perform equally. Manufacturers sell factory-overclocked cards so you don't have to overclock them yourselves. However they are much more expensive and generally not worth the extra money if you are going to overclock them yourself.
Here's a general tier of good sub $200 cards:
8800GTS 512 > 8800GT 512 > 9600GT 512 > 8800GS > 8800GT 256
if your budget is $200 dollars I would go with this 8800GTS 512 ($195 after MIR):
However, the brand is MSI, not the best brand out there, but it is a reputable brand. I don't know much about their warranty and customer service though.
For a trusted brand, and a cheaper card that still performs VERY well I would choose the EVGA 8800GT ($160 after MIR):
They are very cheap right now. It is a great time to pick one up. And also EVGA is a fantastic manufacturer. They have a lifetime warranty, one of the best customer service/RMA departments, and a 90-day step up program.