On such a Low-End Graphic cards, Memory won't really matter much, since you would only be able to play at 720p.
GPU Power = More performance at raw graphics. (And GPGPU Computing)
Memory Bandwidth = Better performance at High Resolution Games. (And AA)
In general, an 5550 should be ~150% more performant than an 6450.
So in short, the 6450 is useless in games, benchmarks tell that the 6450 only can play latest games at around 6 Fps, compared to the 5550 that can achieve at around 20 Fps.
You can also try to buy the 6670, it is an low-end graphic card that perform really well compared to other low-end cards, it can even achieve 40Fps in BF3 at medium settings.
Hope it helps!
Also, if you Only can buy the 5550, try to buy the GDDR3 512MB version, 1GB is useless for low-end graphic cards, even for the 6670.