The FX3450 is a 6800GS in disguise, same clock speeds of 425/1000 as a real PCIe 6800GS and it does compare well to the 6800GT in benchmarks, its got the same size and speed of ram, just a tad less raw fillrate. But you'd have to clock it beyond its limits to reach 6800 Ultra preformance
6800GS fillrate 12x425 = 5100
6800GT fillrate 16x350 = 5600
6800 Ultra fillrate 16x400 = 6400
Now a 6800GT can easily hit 400mhz core, never seen one that wouldn't but for the 6800GS/FX3450 to do so requires a 534mhz core, and while the NV42 core will overclock the limits without voltmods was around 450-475 or 5400-5700 in fill rate.
You can get a 6800GT equal, but the 6800GT will always be faster if overclocked, but you won't get the NV41 or NV42 to hit the same raw fillrate.
As for the stock 350/700 6800 those things are basically more power hungry 6600GT's in preformance, as the extra memory bandwidth doesn't come into play until 1600x1200 and in contemporary games niether card had the fillrate, though AGP 6800 Cards could sometimes unlock to 16pp, the PCI version is still the NV41 130nm chip so its hard limit is about 350-375mhz, I think the absolute latest ones made may have the NV42 that can do more, but you still have 700mhz DDR1.
Ideally look for a 6800XT with 256bit ram, they show up from time to time in an affordable price range, and while they are 8pp, they are often on the NV40 core for AGP as they where trying to get rid of them when the 6800XT released, it came out to dump NV40/NV41 cores right before 7600GS. That could be a good buy, as often they still have 128 or 256mb GDDR3 at 1ghz. Because the only other cards that used it where also out of production, and they had stockpiles of the stuff. Other cards that used it for Nvidia where 6800GT/Ultra/GS/6600GT all where EOL'd at this point, but the 6800XT was made to clear core and ram stock for the newer cards. Was a great buy new and like i said I still see them on ebay from time to time.
And you can flash them to a 6800 or 6800GT depending on how it unlocks or just add 6800XT to the INF file, its supported