hm..worth payin for the extra cache? i don think so. but its a bit faster at least on paper, i mean it depends on what u r doin with ur Pc. U may squeeze some extra fps out of it in games..
(though not u exactly since u only have a 6600gt and that gpu is a real bottleneck and btw forget that thing about SLI, u´ll end up cheaper and with a faster card if u just buy an new and up-to-date single card like a x1950pro or sumthin...well i lost my thought here...uhm..
yea..and if u are in hurry while encodin media and stuff u may go for the E6600 since it might speed up some processes some seconds. But in case all u want is an fast CPU for a fair price then the E6300 or E6400 fits the bill. I dont think most ppl would be able to recognize the difference anyway since its only like 300MHz to those E6600. And in like 2 or 3 years all those CPUs are out of date, so my thinking is, go cheap now and spend the money for somethin else, those E6300/E6400 still offer some good value and horse power.
my2cts
ahh...and for that memory, since u have ddr2-533 there is no need to get new memory unless u desperately want to oc, cuz with that memory u cant oc sh!t cuz its 266Mhz memory which runs 1:1 to the 266MHz Conroe Frontsidebus. With ddr2-667 (333Mhz) memory u would achieve depending on the Conroe model, lets take a E6300 with its 7x multi..7x333Mhz 2330MHz, E6400 8x333MHz, or with DDr2-800 (400) 7x400...MHz...u get the idea..
and another hint word is prices are goin to fall in January and there will be some new cheap CPUs...dont know exactly what will happen but just so u know and can chose to wait or against waiting and buy now...