I'm interested in HD Video editing and not gaming.
Please let me know will ATI Radeon 7750 DDR5 compatible with Intel Board DH55TC ? Will this combination help me in my work? What will be the other requirements to make this best?
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It's compatible, since the only difference between "PCI-e 2.0" and "3.0" is the bus speed, not socket size.
You can plug "PCI-e 3.0" video card into "PCI-e 2.0" slot and it would work just fine.
Socket size-wise there is absolutely no difference between "PCI-e 2.0", "PCI-e 2.1" and "PCI-e 3.0".
Also, if you want quality video editing (by means of speed of encoding/decoding/processing final result) you should think about Lucid Logix "Virtu" support, not about video card support.
For "Virtu"support you need to have a motherboard on a Z68 chipset, and you have H55, so basically, your motherboard doesn't support neither Virtu or VirtuMVP (newer version).
"H" chipset could into integrated graphics of your Processor (Intel HD 2000/3000/4000 and etc), but cannot into OverClocking and there is no support for Virtu (because "H" is too old).
"P" chipset could into OverClocking (you'll be able to OverClock your processor, if it's the "K" line, the unlocked), but absolutely cannot into integrated graphics, again - no Virtu.
"Z" could into both integrated graphics and OverClocking, and has full support of Virtu (not all "Z68" motherboards, though. Some Gigabyte boards can't). Look for "Supports Virtu" info).
To get the very best results during your video editing, you should have: Z68/Z77 motherboard with the latest BIOS, "Sandy Bridge" i5/i7 or "Ivy Bridge" i5/i7 processor, "PCI-e 2.0/3.0" video card with the latest drivers (better to download them online, than to install from the disc), "Virtu/VirtuMVP" installed and running in "d-mode" (don't use the "i-mode").
About Virtu, it's working modes and it's working process:
How to prepare your computer for Virtu (If I'm not mistaken (haven't done that in a while)):
1. Uninstall your discrete GPU and delete all (even last bit of them) of it's drivers. Completely pull it out of your motherboard.
2. Uninstall (If it's installed) Virtu and all of it's components.
3. Connect your monitor to the motherboard.
4. Install Virtu.
5. Plug your discrete GPU into the motherboard and install it's latest drivers.
6. Turn Virtu ON and choose d-mode.
"D-mode" is when your discrete GPU works in an unison with your CPU's integrated GPU, in parallel (which can't be done if you don't have Virtu installed, because in that case your integrated GPU would turn itself off completely when you insert discrete GPU in your motherboard...it's a little Intel's fail, actually, lol), but your discrete GPU is the leading one in the process, thus performance and speed gets quite a lot higher during video processing (for example, in Sony Vegas Pro, during editing, encoding/decoding, re-sampling, and final processing of the edited video), and videogames lose only 1~5 FPS (usually).
"I-mode" is basically the same, but unlike the d-mode, in this mode your CPU's integrated GPU is the leading one, not your discrete GPU. This mode heavily lowers overall FPS of your entire desktop (both games AND all other applications), usually up to -11~13 FPS off of your usual FPS, but video encoding/decoding becomes even faster. It's very unsuitable for games though, because it has strange restriction formula working for games, during it's usage.
This formula is described in such way: no matter which kind of game you play, while i-mode is on, the played game will always get it's FPS lowered, up to 15% of the maximum possible FPS that it could've get during standard non-Virtu usage of the discrete GPU.
To put it simply: if, for example, you were playing Freelancer while only using your discrete GPU for graphics, without Virtu at all, and at some certain point in the game you got 100 FPS, with Virtu in i-mode you would get only 85 FPS at the exact same point with exact same hardware and settings.
It sounds absolutely crazy, but it's true. At least, it was like that during first versions of Virtu, I don't really know about latest versions.
Aside from that - i-mode is still very glitchy, buggy and overall just simply raw.
Both "d-mode" and "i-mode" are settings that you can simply turn on/off in the Virtu's GUI.
On not sure where you got the whole virtu thing but it's not needed and this:
" (which can't be done if you don't have Virtu installed, because in that case your integrated GPU would turn itself off completely when you insert discrete GPU in your motherboard...it's a little Intel's fail, actually, lol)"
Is wrong. All you have to do is go into the BIOS and turn the integrated GPU back on. It's been this way a long time as it's be pretty silly to have a gpu you can't use. There's three choices, always enable(even when not primary), always disable, and auto.