Intel 820, P3 and RDRAM help

[Airkungfu]

Back in March i managed to purchase from my place of work [PC World in UK]a new PC to replace my old k6-2 system.
The spec is P3 1ghz, DVD, CDRW, 128mb RDRAM, 40gb HD and Geforce 2 GTS.

My problem is upgrading the memory..obviously right now RDRAM is expensive but coming down in price....if i decide to buy more does anyone know what type i should buy. All the different speeds available and talk of "must be installed in matching pairs" is confusing me.
At the moment I have one 128mb RIMM installed...it is clocked at 356mhz in the bios...does this mean i have PC700 RDRAM and is PC800[which is the speed most available] incompatible with PC700?

 

[tigerbait]

you do not need to buy in pairs for the 820 chipset. But if you have a pc700, and add a pc800, they both will run at pc700. But that is probably the best thing to do since PC800 is more abundant than pc700 nowadays.