Which RAM should I choose between the two?

Well I want to get a new set of RAM for my PC, but which one should I get.

This one runs stock @ 2.1 volts but its the SLI edition. Also the timing is 5-4-4-15
http://www.newegg.com/Product/Product.aspx?Item=N82E16820227269

document.write(neg_specification_newline('5-5-5-18'));This one runs stock @ 2.0 volts and according to newegg is used in high performance apps. The timing is 5-5-5-18
http://www.newegg.com/Product/Product.aspx?Item=N82E16820227199

Also, I'm running 2x1 gigs of DDR2 800 already in the system @ 888MHZ. Is it safe to leave them there and run these in slots 3&4

(Ive got 64bit Vista so I won't have a problem with the OS recognizing the RAM)