I have a dimension 4550 also and I've gotten 1280mb of ram (1gb stick and 256 stick). You don't need more than a gig on that machine. I installed a 7600gs and I believe the cpu to still be bottlenecking that graphics card. I also had to modify the case to fit a new psu into it.
I doubt that you will be able to fit the 3850 in that case. My x800gto that I killed barely fit.
So, your best bet is to upgrade the ram to 1gb and buy a 7600gs or something similar. You will not even fully use whatever new tech you put in it since it is soooo old and limited anyways. I wouldn't put more money into it. You'd be better off buying a cheap computer anyways or building a new one for $500. But if you want to add more parts to it, that's your decision.
Personally, I haven't even fully used the upgrades I put into mine, just the 1gb of ram and the extra hard drive, that's it.
Let me know if you have any other questions about it. I've done tons of research on this system so I know it inside out.
so my question is, whats the best that i can make my dell dimension 4550.
I'd say make it into a file server, or internet/web machine is your best bet. You can only play games (around 2003) very well on it anyways, so new games are out of the question. I think when I can get my parents off of it, I'll hook mine up to my tv and play with around with that as a media center sort of...