Replacement components for a computer that does not post

My desktop computer has been exhibiting unusual behavior over the pass week. Unfortunately, my pc does not always post when the power button is pressed. Sometimes, the computer turns on after several hours. When the computer does not post, I cannot hear the drives spin up or any posting process (including beeps).

Unfortunately, my time is limited for researching and troubleshooting the situation since this computer is down. I have re-seated all the components inside the case, yet do not have additional components to swap in and out. The computer switch seems to work on the case. When opening the case and pressing the power button, CPU, graphic card, and case fans turn on.

My initial thoughts relates to the motherboard or video card since the power supply is less than 6 months old (Corsair TX750W). When the computer turns on, it does not encounter any errors or hiccups; and performs as expected.

My first inclination is to begin looking into replacement components, yet I was not planning a major system upgrade. So I humbly request suggestions on replacing my motherboard, processor, and video card. My current system has processing power for most computing tasks except when doing heavy multi-tasking with streaming HD video. My requirements for replacement components would revolve on a dual monitor setup; moving large files, file archiving, and HD video decoding in a multi-task environment. This rig does not serve as a gaming machine. Value is a top priority for replacement components. My budget will be between $300 and $400.

AMD seems like the better value. The Phenom II X6 1090T looks good with 6 cores. The motherboard and video card are not easy choices. It has been 5 years since my last system build and the technology has rapidly evolved. The motherboard should have at least 6 Sata connection (most seem to have 4) and on-board sound. USB 3 would be nice, yet not essential. The video card should be powerful enough for basic photo editing and viewing HD content. I do have 4GB of OCZ PC2 6400 ram from a previous hot deal, yet did not work under my current setup. Below is a short list of potential candidates for replacement component and my current system.

I am hoping this thread would begin a discussion on the components that offer the best bang for the buck. Any suggestions would help me avoid major disappointments with getting this systems back online.

Update: the computer has not turn on in several days.

Replacement Components
-----------------------------------
Phenom II X6 1090T Black Edition
ASUS M4A88T-V EVO/USB3 AM3 AMD 880G HDMI USB 3.0 ATX AMD Motherboard
XFX Radeon HD 4670 Video Card or something under $150

Current System
---------------------
Processor: Athlon 64 X2 4400+ (2.2GHz) 1MB L2 Cache Socket 939
Heatsink: AMD Heatsink-Fan: Thermalright XP-90C w/92mm Fan
Motherboard: A8N-SLI Deluxe
Video Card: XFX GeForce 7800 GT 256-MB PCI Express
Power Supply: Corsair TX-750w
Memory 2x1GB 400Mhz PC3200 DDR RAM 184PIN DIN
Monitor Dual Dell 22 Inch 2209WA

Well if u stated video editing, etc on the table then i would say yes an X6 and 8GB RAM would be in order but for the above tasks u dun need to splash on such high end stuff ^^ Also the reason DDR2 didn't work on your current set up is due to socket 939 Integrated Memory Controller (IMC) being strictly DDR only hehe

So if Microcenter were an option i'd just reuse those idling pair of DDR2s i.e

X4 840 + MSI 785GTM-E45 AM2+/DDR2 mobo$99.99
http://www.microcenter.com/special [...] PROMO.html

The HD 4200 IGP can already do dual monitors set up for non gaming, it's predecessor doing dual/quad displays back in '08

Toms dual/quad 780G display review
http://www.tomshardware.com/review [...] 785-5.html

@GeekApproved

My computer has not started in a week. I am discouraged that it won't come back to life. Changing the motherboard battery made no difference. Today, I pulled out the memory, modem, and disconnected the drives. The power supply could be an issue, yet it is fairly new and a highly rated brand (I still have the box). I don't know about troubleshooting the power supply otherwise. Generally, the computer becomes unstable when other components act up.
I have never had a computer not post for me (even with a bad power supply).

@batuchka
Thanks for your suggestions. You have good advice. Please let me know if a motherboard exists that could handle the DDR2 and DDR3 memory. In the past, I have avoided on-board video; these days the on-board video fairs better. How does on-board video chipset compare to GPUs with dedicated memory (more of a question about video memory)? The 785G chipset would require an additional speciality cable for the second LCD. Let me know if you have any suggestions here (VGA to DVI or HDMI to DVI)

I also came across this memory deal, which seems reasonable to pickup DDR3 (any thoughts). Going with DDR3 will help me find cheap memory in the near future.

CORSAIR XMS3 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Desktop Memory Model TW3X4G1333C9A G
http://www.newegg.com/Product/Prod [...] -_-Product
$49.99

Oh he still has the box for his Corsair power supply. There's no way it could be that.

Really? That's amazing because that's what happens 99% of the time when you have a bad psu, it doesn't power on. LOL

Your randomly replacing parts and you have no idea if that's even the problem. Your gonna get a big surprise when you get your mobo and ram and find out it still doesn't post.

Yeah great advice!! Good luck.

1. Yes 939/AM2/AM2+/AM3 CPU socket layout similar
2. If u hitting AM3 mobo - have a look at that link i gave ya (MSI 870A-G54 bundles)
3. I would say 4GB for your usage patterns would suffice ^^