Perfect GPU for my CPU

Hi there! I have a Pentium(R) Dual-Core CPU E5700 @ 3.00GHz processor in
a motherboard GA-G41MT-S2 with 4 g of ram and POWER Supply of 300
wats (a cheap one). I am planning to get a GPU, the HD 7770 indeed, but a friend told
me it was to much for my cpu and that it will not take out all the
potential of that GPU. I will get another power supply if it is
necessairy, but I wanna now what gpu will be the best for me in my
actual settings (changing the power supply of course). I wanna play games in normal settings and make basic rendering. I saw a HD5450 in my local store, will it be better for me rather than the hd 7750 (o 7770)? Please help me, I dont want to waste my money