Are you sure it is the 3870 that is causing the crashes?


In any case I would look at the GTS 250, but to get your moneys worth i'd try to go for something faster like a 5770, GTX 260, or GTX 460 when that comes out.

This isn't an answer to you question exactly, but it may help.
(Links are underlined)
I have Sapphire's two slot version of that card. Performance aside (which a new card would surely improve on), the lock-ups are likely due to ATI's "Powerplay" garbage. (The fan speed in particular) When I first got the card I was noticing that under load, my gpu temps were getting close to 80-90C, but the fan was only running at 40%.  So I went here and did a little shopping.

IF YOU AREN'T COMFORTABLE WITH THE IDEA OF BIOS FLASHING OR EDITING STOP HERE! Improper use of BIOS editing and flashing tools can turn your card into a PAPERWEIGHT! Proceed with caution!

I downloaded RBE (Radeon Bios Editor) which is a pretty staight forward tool for doing just what the name implies. It lets you change the programming the card uses for temp/fan speed ratio, as well as gpu and memory clock speeds/voltages.

I also picked up ATI WinFlash so I could flash my card with my new and improved BIOS.

All I did was eliminated any temp/fan speed curve, and made the BIOS of the card spin the fan at 100% from out of the gate. (You may not want to do this if noise is a concern. My card now sounds like a hair dryer, but since I use headphones anyway, I'd rather have a loud card that's cool, than a hot card that's quiet!)

I also addressed the other aspect of "PowerPlay" and completely did away with the 300Mhz core clock idle speed. So now my card runs at it's maximum ADVERTISED clock speed all the time. (I did not oc the BIOS since Catalyst lets you do this from the overdrive control panel anyway) Now the card doesn't throttle when I'm playing older games like FS9 (which would rarely tax the gpu enough to get it ramped up from it's 300Mhz idle speed, resulting in agp performance levels), it just gives me full speed all the time. No lag, no stutters, no ramping up and down of the clock speed. It's nice.