I've played Doom 3 on seven different systems now, the lowest of which being a AXP 2400+ with 1024mb ram and a GeForce 4 MX480, the most powerful of which being an AXP 3200+ with 1024mb ram and a Radeon 9800 pro.
Realistically, unless you have some of the very top-tier cards, such as the 9800 PRO/XT, the 5700 Ultra/5900/5950, et cetera, you are going to be sacrificing either framerate or quality, your choice!
The 5200 really is a crap card for 3D gaming. It holds no real advantage other than hardware DX9 support and improvements to a few things like pixel shading. You aren't going to get much out of that, especially in Doom 3. It will probably even be worse with Half Life 2.