Hey your system can handle the hd 4650 but i think hd 4670 will be a little difficult due to your 250 watt psu.... if you can replace your psu then you can use any card you want as 250 watt is little less bro.
get the hd 4650 it will be great for these games and at 1280 x 1024 resolution it's the best i can suggest you....this card can handle more games too on higher settings that are more gpu intensive then the games you have mentioned.
And no need to change psu too.
It's right the 512mb vs. 1gb video memory isn't going to be much of the noticeable ...so either one will be good but going with 1 gb will help you in increasing the resolution if you want to in future.
I have this exact comp (well almost, mine is a Compaq CQ 5619F but with exact same specs) and the same issue; I'm investigating what vid card to get for it and I'm surprised that it can be done without changing the shitty 250W psu.
The specs on my psu are:
LiteOn model PS-5251-08 rev. 04
combined power on +5V and +3.3V rails not to exceed 165W Max.
combined power on +12V and +5V rails not to exceed 218W Max.
Output: +5V --- 25A Max. +12V --- 14A Max.
+3.3V --- 18A Max. -12V --- 0.8A Max.
+5Vsb --- 12V Max.
I guess model 'PS-5251' stands for pos
Anyway I used this power supply calc with the Radeon HD 4650 and the total comes to 231W but that's only a recommended minimum psu wattage so is it realistic to use this vid card with this psu? Seems like it's cutting it very close and what would happen if the power went over 250W?
BTW were can I find the physical measurements for vid cards so I know if it'll fit?