pc800 its rdram or ddr2

184pins=rambus
240pins=ddr2
184pins=ddr

but u cant find pc800 ddr memory 400MHz its posible only on rambus or ddr2

Nope it seems thar you need RDRAM PC800 .

[code:1]http://www.4allmemory.com/index.cfm?fuseaction=search.memorySearch&model_id=17280[/code:1]
[code:1]http://stores.tomshardware.com/search_getprod.php/masterid=394215[/code:1]

Tech Specs:
Package Type: 184-pin RIMM
Package Type: 184-pin RIMM w/ECC PC800
Logical Type: 256x18
Logical Type: PC800
Speed: N/A
Speed: PC800
Description: 512MB Upgrade for Dell Dimension 8100 Series
Description: ECC, Unbuffered
Units Required: 2
Install Notes: Installed in pairs of modules. Please order quantity two.[Installation Tips]
System Type: Desktop/PC

From the horse's mouth:

Memory
Architecture RDRAM
RIMM sockets four
RIMM capacities 64-, 128-, 256-, and 512-MB RDRAM
Standard RAM 128 MB (minimum)
Maximum RAM 2 GB
BIOS address F8000h

You have to buy compatible RDRAM but it doesn't have to be from Dell, but it's fairly rare these days so not a lot of places sell it.

I've already given you the motherboard specs. You don't have 2 types of slots, just RDRAM. Dell's in the business of saving money wherever possible, so they wouldn't waste resources by adding different memory slots and most likely a second northbridge or some convoluted solution. I've never heard of a dual RDRAM / DDR board, doubt it's easy to do.

I'll say again:

From the horse's mouth:

Memory
Architecture RDRAM
RIMM sockets four
RIMM capacities 64-, 128-, 256-, and 512-MB RDRAM
Standard RAM 128 MB (minimum)
Maximum RAM 2 GB
BIOS address F8000h

You have to buy compatible RDRAM but it doesn't have to be from Dell, but it's fairly rare these days so not a lot of places sell it.