Which runs better for me, HD3850 or 9600GSO?

Those two cards are virtually dead-even. However since you're only running at 1024x768 resolution, the 9600GSO might have a slight advantage. For higher resolutions I'd take the HD3850. In this case it's really a toss-up- go with whichever brand you prefer.

Update - XFX is a very good brand!

Some benchmarks are here:
http://www.guru3d.com/article/geforce-9600-gso-386-mb-review-point-of-view/1

The 9600gso beats the HD3850 in some things, is a bit worse in others. For Crysis, the two look to be exactly even (at least, according to this one article).

From what i've gathered the 9600gso would be the stronger of the two cards for the game in question. It even outperformed a 3870 and a 9600gt according to these benchmarks in Crysis and a few others. The price range of the cards is what it really comes down to, as the only thing the 3850 has going for it is more vram, which at a resolution of 1024x768 will make no difference. Both cards offer excellent value for performance, and i think you would be happy with either one. That being said I prefer Nvidia cards from EVGA or XFX.....both companies offer great warranties. The transition will be easier as you already own an Nvidia card.....I have actually had to run repair installs because of driver conflicts when switching from ATI to Nvidia and vice-versa. There are utilities to take care of that problem now(driversweeper from guru3d.com).....but just be aware that the potential for a problem exists. Whatever your choice i hope you enjoy Crysis.....truly a benchmark game!!! Hope this helped some.....

I'd prefer 9600GSO.

9600GSO is by NVidia, NVidia is known to be better than ATI for games.

Because on how the games today, they prefer their games to be ran by NVidia.

And yes, I'm pretty sure you can play Crysis on medium settings with it.

Yeah, XFX is good.