a 5770 would do very well, if its within your budget. if thats too much, a 5750.
right now though, ATI offer the best performing cards for the best price in almost every sector. a 5 series card is almost certainly the way to go, so id say grab the best 5xxx card you are willing to pay for.
Can you define "medium" price tag? An 8800GT/9800GT and the HD5670 are all about $100 and all have the same performance so you'd have to spend more than that to get any noticealbe improvement. Also, what size monitor and resolution?
That said, the bigger problem you're going to have is that your X2 is going to have trouble pushing anything more than what you've already got. I built a 6000+ overclocked to 3.4GHz, and it couldn't push a 4850 to save its life and the 4850 is about 5-10% faster than those. You COULD step up from those to, say, a 5750 (which I've been able to push to dang near 4870 benchmark scores using a X4 965BE), but you'd be at $130 and you wouldn't come close to stressing the card. The 6400+ becomes a bottleneck around the 4830 and 9800GT. If you need a new card anyways, go with a 5750 or a GTS250.
...have problems with white gliches and it overheates turning my computer off.
What, specifically, is overheating? What mobo do you have? Does it [really] support 125W CPUs? How is your case cooling, is there a good flow through the case, or are all your fans exhausts? What brand and model is that "750W" PSU? Unless the thermal problems are specific to your GPU (e.g. a failed cooler on it), getting another GPU won't alleviate your problem, and may make it worse.