Yes theres a big difference. The generic Ram are at those timings and mhz and will not go much higher. On Value DDR2-800 ram you will be lucky to Reach 430mhz ever. While on quality ram you can reach speeds of 500+
It all depends on the ram and price. Just do your homework. Also there arent many what you would call "Generic/Value" DDR2-800 ram. Most value is at 533 and 677. You will find DDR2-800 that reach alot higher speeds for the most part but my G.Skill was cheap ram and it reaches 540mhz+ Ram at 5-5-5-15
EDIT: Now if you mean are generic BRANDS any different it depends. Most ram these days are fairly close in the lower end. If you buy lets say Wintec or PQI or something that has a less known name it doesnt mean its bad ram, it's just not as known as the others and to keep competitive they put it at a price where people will notice them and buy it. Although of coarse ram with better names will on average make better ram. Unless your building a super high powered system you could probably go with cheaper ram and be fine.
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