the GTS 450 is the better card and the better buy for your money. fallout 3 is not a very demanding game though so you will have no issue with the GTS 450. you probably won't see the full potential of the card though with your CPU.
the nvidia 8400gs would fall below the ATI 5450 in the chart below in terms of performance
1) you would not see a difference with the AMD 6950 and Nvidia GTS450 while using a pentium D CPU. while the AMD 6950 is obviously the better performance card it will not double your FPS, it will in fact barely change it.
let's look at actual facts rather then stupid opinions
the OP has a Pentium D but in this benchmark they use an athlon x2 5000. see passmark link as rough benchmark, the athlon x2 5000 obviously should offer slightly better performance. http://www.cpubenchmark.net/common_cpus.html
with the athlon CPU the ATI 4850 (similar performance as the nvidia GTS450) just stays under 43FPS yet with more powerful cards the like the GTX 285, the FPS only increases to...45FPS. The video cards are clearly held back by the CPU since with more powerful CPUs, the more powerful video cards are able to accelerate FPS. Now, with a CPU that is less powerful then the athlon x2, do you think the difference in video cards will be bigger or smaller....
2) upgrading a CPU on mass vendor is not an easy thing as you are limited by the vendors BIOS rather then the mobo manufacturers, helps increase sales by limiting upgrades. the GX620 was sold only with P4, PD, and celeron processors so 99% of the time with DELL those are the only CPUs that mobo will support. So the OP is already at the top of his food chain with a PD @ 3.4ghz.
3) older DELL mobos that use PCI-E x16 1.0 slots like the the OPs have issues with PCI-E 2.1 cards (most modern AMD & ATI cards). once again its an issue with the DELL BIOS. 2.0 cards (like modern Nvidia) don't have this issue with Dell mobos.
perhaps you should read more on the subject of PC hardware to help understand it better before offering any more foolish & "pg-headed" opinions.