Looking for some mobo help

[oneil]

I bought a refurbished comp last boxing day, and have been upgrading away at it ever since. After putting in the new power supply recently, I realise the biggest problem with my system is the motherboard. Up to then I didnt worry too much about it but it has become such a bottleneck I'm now in the market to replace it.

Here are my system specs...
AMD Athlon 64 3500+ (Socket 939)
PCI-E BFGTech 6600GTOC [625/1200]
1.5Gb PC2300 RAM (3x512MB)
Soundblaster Audigy 4
ATA IDE 250G Maxtor (1x)
SATA 200G (1x)
OCZ PowerStream 520W ATX12V
HP DVD-ROM
HP Litescribe DVDR-RW
Watercooled (specs in sig)

I dont plan to upgrade to SLI, but the extra PCI-E is welcome. I'm looking for something that wont bottleneck for years to come, whilst still being compatible with my current hardware. My case is an Aspire X-Dreamer II (click for pic of current inside)

Budget is not an issue as of now, but would obviously prefer something on the resonable end. Will post research in a few - work is keeping me busy :|

 

[oneil]

ASUS A8N-E -> $115
ASUS A8N-SLI -> $129 (Does the 4pin molex on the mobo for PCIE help much?)