NotGoodWithComps

OK so heres the deal. I currently have 512 MB of RAM installed on my comp. I have 4 slots in my computer for RAM. Two are empty, two are filled with 2 x 256 sticks of DDR RAM (Atleast I think). The RAM sticks that are currently installed appear to have 1 notch near the middle. Now, I went out and bought more RAM today that was the same as these, but then found out they do not fit in the 3rd and 4th slots because there appears to be a second spacer in these slots. Now, I am guessing this is for SDRAM or whatever it is called. Now my question is this, Can I run both types of RAM? I want to buy 2 more 256 sticks of this different kind of RAM to upgrade my computer to 1 gig. I read the manual of my motherboard (ECS) and it says "you can only use one of the both SDRAM and DDR SDRAM at the same time". But, the picture shows only two slots, not four like I have. So I am asking,

1. What RAM should I buy to fit in these different spaces
2. Will my computer be able to work with both types of RAM running at the same time.

My computer specs are:
Operating System: Windows XP Home Edition Service Pack 2 (build 2600)
System Model: = ECS = P4VXASD2+ = 1.0
System Serial Number: = '00000000'
Enclosure Type: Desktop
Processor:2.40 gigahertz Intel Pentium 4
8 kilobyte primary memory cache
512 kilobyte secondary memory cache
Main circuit board: Board: ECS P4VXASD2+ 1.0
Bus Clock: 133 megahertz
BIOS: American Megatrends Inc. 07.00T 04/02/01
Drives: 80.06 Gigabytes Usable Hard Drive Capacity
57.83 Gigabytes Hard Drive Free Space

CyberDrv CW088D CD-R/RW [CD-ROM drive]
JLMS DVD-ROM LTD-166S [CD-ROM drive]
3.5" format removeable media [Floppy drive]

SAMSUNG SP8004H [Hard drive] (80.06 GB) -- drive 0, s/n 0415J1FTB23904, rev QW100-60, SMART Status: Healthy
Memory Modules: 512 Megabytes Installed Memory

Slot '0' has 256 MB
Slot '1' has 256 MB
Slot '2' is Empty
Slot '3' is Empty

Bus adapter: VIA Rev 5 or later USB Universal Host Controller (3x)
Multimedia: MPU-401 Compatible MIDI Device
Standard Game Port
Vinyl AC'97 Codec Combo Driver (WDM)

And here is some help from the ECS manual...

Installing Memory Modules
This mainboard accommodates 168-pin 3.3V/184-pin 2.5V unbuffered
SDRAM memory modules. The memory chips must be standard or registered
SDRAM (Synchronous Dynamic Random Access Memory).
The CPU supports 100MHz system bus. The SDRAM DIMMs and DDRs can
synchronously work with 100 MHz or operates over a 133 MHz system bus.
DDR SDRAM provides 800 MBps or 1 GBps data transfer depending on
whether the bus is 100 MHz or 133 MHz. It doubles the rate to 1.6 GBps and
2.1 GBps by transferring data on both the rising and falling edges of the clock.
DDR SDRAM uses additional power and ground lines and requires 184-pin
2.5V unbuffered DIMM module rather than the 168-pin 3.3V unbuffered
DIMMs used by SDRAM.

(I don't know really what any of this means, so please fill me in)

ANY HELP WILL BE GREATLY APPRECIATED!

Thanks a lot.