discrete means, independent. Integrated video chips use computer’s “main memory” and computers main CPU to process data while a discrete Video card comes with its own CPU and RAM. The CPUs on video cards are specially designed to serve the purpose.
So, 512 MB of ATI HD 5870 will out perform 1GB of ATI HD 5470 easily.
but 5470 should be arite to work on photoshop just dont go for any thing less than that.
go for a business class I say, VOSTRO maybe