Ultimately, it depends what resolution you play at. The more memory on the card, the easier the card will handle high resolution. If your monitor does not support 1920x1080 (or greater) the higher ram won't be utilized. DDR type is important, and generally newer is better (e.g. DDR5 is better to have than DDR4) but different cards play different games better as well.
DDR type is important, and generally newer is better (e.g. DDR5 is better to have than DDR4) but different cards play different games better as well. Another factor is noise, not sure if that matters to you or not, as well as length, since I have a mid-tower case and a Radeon 5970 won't fit in mine, for example. Another item is power. The top-flight cards take two PCI-e 6 pin + 8 pin connectors (for one card!).
the 5850 usually averages about $50 higher. The Radeon is nice as you can fit 3 monitors to it (not sure if that matters to you).
Butt, I guess the ultimate question is: what's yer price range??
If you can swing the 5850, that will be enough card for you for a couple years, on top of being able to handle full 1080P playing resolutions should you upgrade your monitor. But at the same time, you can argue that you when you get your 1080P monitor, the 5850 will be cheaper too.That said, if you're not a sucker for eye candy (personally, as much as I like graphics, I just don't like jagged lined - once the jagged lines are gone, the excess AA/AF and field of view are less important to me), the 450 at $90 is very good deal.