What ram actually is that.. 'High Quality Grade Memory for Dell Laptops' ?

your XPS based 965PM, so the highest mem. speed it runs is 667mhz Max.
although if you go for pc26400s-800mhz, it'll still downclocks to 667mhz.

So, the best option you can pick is,
go for low-latency ddr2 sodimm, aka tight timings.
there are 2gb stick sodimm 667mhz rated @cl4
get 2 sticks of them and run it @ 2x2gb dual channel 667mhz cl4.

normal ddr2 sodimm-667, is rated @ cl5
so the LL kit with cl4 will have higher bandwidth thus better performance.
but do note that, the performance difference between both is very minimal.

dude p965 supports only 667mhz rams..

so having a performance ram not worth it..
but if u want.. u can order from cycom
for kingston hyperx pc2-6400 sodimm