To be perfectly honest, I wouldn't do so. I had an XFX Radeon HD 4770 myself, but gave it away to my friend as a gift, it was a very nice card in terms of power consumption and performance for its price, but since you have an EVGA GeForce GTS 250, I really see no need in exchanging it.
40nm cards will be clocked higher to reach higher performance levels while sticking to similar, if not slightly lower thermal profiles as cards on older fabrication processes.
If you want 40nm then go for it, but what the majority of the responders are saying is to just not bother and enjoy the GTS 250 for now, and wait until a more worthwhile upgrade comes along.
Because your case is 'congested', though i think based on what you've said the better term would be 'cramped' if it's a small case, then i would say that you won't be able to accommodate larger length cards typically found in the higher performance tiers.
If i were you, if the GTS 250 is working great and not overheating, then just leave it in there.