What you should be looking on the cards, is shader processors (SPs), what kind of memory it's using (GDDR) 5 is currently in the high end cards, higher is better ofcourse.
Third thing is memory bandwidth, its measured in bits, and higher is better, currently fastest single GPU is Nvidias GTX 480 which has 480 CUDA cores (Shader processors) and 384 bit memory bandwidth and it runs on GDDR5, now GT 220 has only 48 SPs and memory bandwidth is only 128 bit and uses GDDR3, you can see the difference is huge, GT 220 is no gaming card really, it's low budget casual gaming or just generic GPU that can handle fair amount of stuff.
What you need to remember is that ATI cards use different technology and they've got generally more SPs on their cards which doesn't translate to more power, so you can't directly compare by SPs between ATI and Nvidia.
Now we know GT 220 has 48 cuda cores (not relevant when going against ati) 128 bit memory bandwidth and GDDR3 with 1GB of memory. (also core clock and shader clocks affect directly to performance, they can't be compared because different GPU can run at same speed but isn't nearly the same).
When we look at ATI's 5750 first thing we notice is that it has GDDR5 (big plus).
Second thing we notice is it has Direct X 11 support, which is nice when some dx11 games start rolling in that aren't designed for high end PCs.
It also has 128 bit memory bandwidth.
ATI 5750 is definitely the winner here but you'll see that in price tag too, GT 220 costs around 60 euros around here and ati's card is around 140's.
There's some basics so you can compare some cards and pick the one that suits you without getting fooled by the model names which sometimes are confusing.
edit: wow god damn this became long post... hope it aint too long and somebody actually bothers to read it
edit2: something about the amounts of memory:
the memory size in most cards is overkill when comparing it to their performance, most games run just fine with 1GB, but some like GTA IV can require more if you tweak the settings like so, but theres no way you can even run that game with GT 220 or even with 5750 in such high settings so that VRAM (memory) would get filled, basicly, more memory won't give more performance, that's something many "noobs" are confused about and that's one thing that stands out from the cards when you're shopping, but it's not relevant at all to measure how good the GPU is.