well I got 768mb of PC2700 RAM that's OC'd to 372MHZ from 333, and I'm using a FX 5700 256mb card, just the regular not LE, or Ultra or anything, my CPU is 2.6GHz but I'm guessing you have an Intel whereas I have an AMD so Ghz to GHz and AMD will beat out an Intel so technically I have a little more processing power more than likely, and I run doom on High Settings, noaa, noaf, 800x600 Resolution, it's playable, but will get laggy when monsters really start attacking you, so with that setup you currently have you'll probably have to do like medium settings because the 5200 is not at all a gaming card, it's a 'mainstream' card just like the 6200 will be, whereas the 6600 is the 'performance' and the 6800 is the 'enthusiasts' model. You might want to wait for eVGA to release their 6600 in agp because they are a real good quality card maker.
All in all with your current setup doom3 will need low settings and will probably be laggy to a certain extent, perhaps not on complete low settings and low resolution, but I know that 5200 will have a tough time with a game like that. The 6600GT on the otherhand should slice through it no probs, that's whith your current setup in mind