I've never heard of a Tagan PSU so I'd avoid that like the plague. Get a nice brand like Corsair. Quality of the PSU matters WAY more than wattage and it's almost always a bad idea to buy generic or no-name brands.
Your CPU will probably bottleneck a bit... it's pretty old. You should also upgrade to 4GB ram if you can (I know you said that's the plan - just didn't list it on the new rig). Personally, I'd probably stick to an HD 7750 for that rig... I don't think that a GTX 460 will give you much benefit on such an old CPU.
IMHO you are upgrading the wrong parts. What i did was upgrade bit by bit. I used the same case and power supply. Got a new MB, cpu and 1 stick of 4mb ram. I stuck with intel hd graphics til i could afford the rest of the upgrades. I was able to play SWTOR at 60 fps (ya everything low).
going from 450 power supply to one with just 50 more seems like a waste. IMO i would keep the psu and case and swap out the MB to one that can at least use ddr3 ram and the CPU you want later ( i looked for upgrade avenue so i got one with xfire/sli support) grab 1 stick of 4mb ram and you can still use that same graphics card.
This will leave you avenues to upgrades in the future.
Not sure what your budget is but ballparking it:
CPU: I3 2120 around 120$
MB: So many options here but less than 100
This will start you out for a very decent budget rig.