Most Dual Channel DDR2 Kits will work for you. Your best bet is to use Crucial DDR2-1066 (because the SPD is programmed for lower speeds). You can run CPUz but it will only tell you the speed of the ram you have installed and it won't tell you the maximum speed your motherboard is capable of.
I have found an Anandtech Article on your motherboard, here are the full specs:
(I am too new to post a link, so google search: anandtech 2894
ASUS P5N-E SLI Basic Features
ASUS P5N-E SLI
Market Segment: Mid-Range Performance - $129.99~$149.99
CPU Interface: Socket T (Socket 775)
CPU Support: LGA775-based Pentium 4, Celeron D, Pentium D, Pentium EE, Core 2 Duo, Core 2 Extreme
Chipset: NVIDIA 650i SPP (C55) + 650i MCP (nF430)
Bus Speeds: Auto, 533 to 3000 QDR in 1MHz increments
Memory Ratios: Auto, Sync, 1:1, 5:4, 3:2
Memory Speed: 400MHz~2600MHz in various increments
PCIe Speeds: 100MHz~131MHz in 1MHz Increments
Core Voltage: Auto, .83125V to 1.60000V in 0.00625V increments
Core Voltage Offset: Disable, +100mV
CPU Clock Multiplier: Auto, 6x-11x in 1X increments if CPU is unlocked, downwards unlocked, Core 2 Duo
DRAM Voltage: Auto, 1.920V ~ 2.517V in various increments
DRAM Timing Control: Auto, 13 DRAM Timing Options
NB Voltage: Auto, 1.208V, 1.393V, 1.563V, 1.748V
Memory Slots: Four 240-pin DDR2 DIMM Slots
Dual-Channel Configuration
Regular Unbuffered Memory to 8GB Total
Expansion Slots: 2 - PCIe X16 (2 - x8 electrical for SLI or Multi-GPU)
1 - PCIe x1
2 - PCI Slot 2.2
Onboard SATA/RAID: 4 SATA 3Gbps Ports - 650i MCP
(RAID 0,1, 0+1, 5, JBOD)
1 e-SATA 3Gbps Port - JMicron JMB361
Onboard IDE: 2 ATA133/100/66 Port (4 drives) - 650i MCP
Onboard USB 2.0/IEEE-1394: 8 USB 2.0 Ports - 4 I/O Panel - 4 via Headers
2 Firewire 400 Ports by VIA VT6308 - 1 I/O Panel, 1 via Header
Onboard LAN: Marvell Gigabit Ethernet
- PCI Express - 88E1116 PHY
Onboard Audio: Realtek ALC883 - 6-channel HD audio codec
Power Connectors: ATX 24-pin, 4-pin EATX 12V
I/O Panel: 1 x PS/2 Keyboard
1 x PS/2 Mouse
1 x Parallel Port
1 x e-SATA
1 x S/PDIF Coaxial - Out
1 x IEEE 1394a
1 x Audio Panel
1 x RJ45
4 x USB 2.0/1.1
BIOS Revision: Award 0101
Board Revision: 1.01G
