if i remember corectly.. the athlon 3800+ was on a rated scale comparing it to the equivalent intel cpu, or a Pentium D at 3.8ghz. my e5300 and athlon II 250 are about the same performance wise, and i can tell you its much faster than what my Pent. D at 3.5 was. MUCH faster. so with that, the 250 should be a lot faster than your current cpu.
i can also tell you that my athlon II x4 630 was much faster than my current cpu. which is on par with a phenom in some cases at the same clocks, with some apps favoring the extra cache (the L3 is not on the athlons, and some "phenom II")
as for gaming, my friend plays the newer Empire: Total War just fine on his 250 and a GT430 gpu. and my cpu does fairly well with borderlands, mw2, etc.
oh my reasoning about cpu performance comparison... the following links
http://www.cpubenchmark.net/cpu_list.php
^cpu list, pretty complete one at that.
http://www.cpubenchmark.net/cpu_lookup.php?cpu=AMD+Athlon+64+X2+Dual+Core+3800+ Athlon 64 x2 3800+
http://www.cpubenchmark.net/cpu_lookup.php?cpu=Intel+Pentium+D+3.60GHz Pentium D 3.6ghz (i figure 3.8 would be closer)
^ those 2 about the same
http://www.cpubenchmark.net/cpu_lookup.php?cpu=AMD+Athlon+II+X2+250 Athlon II 250
http://www.cpubenchmark.net/cpu_lookup.php?cpu=Pentium+Dual-Core+E5300+@+2.60GHz Pentium E5300
^ those 2 about the same
Edit: looking up your PSU, will edit again when i get infos
info found. ok when using google translate on this site,
http://www.proline.pl/?p=TAC+RADIX+III+420 , I found that you have 2 12v rails, each at 15A. one is probably dedicated to the cpu, so you really have 15A to play with on the other for a gpu. you will be happy to know that the athlon II 250 uses less power, so you're power supply should run more stable when upgrading to a gpu with athlon 250 than if you kept your current cpu. that said... ima subtract 3A to cover hard drive, optical, and fans. so lets say you have about 12A for a video card. that's about 144watts. any gpu you get that doesn't need a pci-e connector you should be fine with, and a few that require ONLY a six pin pci-e will work as well, but some will be pushing it (75w through pci-e, 75w through the 6 pin, that's 150w and you only have 144 best case on that rail). The GT430 would be a nice option (no ddr2 models, they suck) or any AMD equal to that. when upgrading you'll want a newer series gpu, as they'll use less power for the same performance of an older gpu.
of course you can always get a new psu
hope this info helps. if you have questions about a particular gpu you're considering, please post and we'll let you know if your 420w will work with it.