just for sharing,i'd just bought a KVR kingston from a shop in lowyat.they gives warranty to 1 on 1 change to other ram if it's not stable.just did the memtest last night,everything's working fine and seems like no problem until now lah..
btw,the removing battery ribbon only apply to latest macbook pro(the one comes wif internal batt)if older one,i think u just have to remove the battery and it's safe edi.

that part i'm not sure lar...i don't they provide that service for mac.nahh.... just few very easy steps maa...be gently and carrefull in changing the ram,and everything should be ok.

ya'sure..coz u'll be reserving some of ur rams to your XP and yes,having more rams would significantly improves the performance of ur multi OS.

ya,DDR2 and DDR3 can't be use together..but, if u have two DDR2 with different clock speeds,then u can use them together.but the ram will follow the lowest speed between both ram speed.for campatibility issues,better u use same ram lah,coz sometimes diff speed diff clock cycle,might end up crashing and compatibility issues.

i think it's best if after u install ur newly not apple ram one, u do a hardware test. i did like that before, so, if anything problem u can go to the shop and try to change for new one. can get 1 on 1 change before 1 week.