Well I'm planning on removing the stock heatsink on the 4850 since it idles pretty hot like around 75-81 degrees. Since the new heatsinks have bigger fans and whatnot I'm guessing it'll require more power, would my 450W psu be able to handle it?
The size change in fans is not going to require a PSU change. (very little change in power draw). I would buy the cheaper one unless you are planning a massive overclock.
Then again how old is the card? Are those temps with a clean fan and heat sink? Cleaning the fan and heat sink so they are free of dust is a must to keep temps down.
The vram is easy a toddler can do it but the VRM is of concern. You can try to uses several vram heatsinks but that is no guaranty that it will work. The best option requires skill and the materials needed if no ready made heatsink is available. You see those mosfets to the rear of the card that is what you need to worry about. Your situation is better than that of the 4870 and the 4890 owners who are moving to a better cooling solution.