Down grading to XP from W7 compatibilty

Hi, first post so please be gentle! 

I purchased a new PC as my previous dual-core P4 with 2GB ram was having hardware issues. I was running XP SP2 with zillions of apps and games and was fairly happy but decided to upgrade. I was advised (wrongly it seems) that I would need to run W7 64bit on my new hardware so I grudgingly upgraded and was met by dozens of compatibility issues having to upgrade many apps I use daily and lose some that did not have W7 or 64bit compatibility.

After a couple of weeks of running Windows 7 (and struggling to install some of my fav apps!) I finally got around to trying to install some games and found half a dozen so far with compatibility issues. I have tried running various compatibility mode, privilege and other setting combinations and some times these work but mostly I get error messages. I have spent the whole evening reading page after page of similar stories and various possible solutions but resolving all my issues looks like a hell of a lot of work that I don't have time for! I really don't see any personal benefits to Windows 7 for me and want to go back to my trusted WinXP but using this new super fast PC.

My personal preference at the moment would be to revert back to Windows XP. Either in 32bit or 64bit guise.

My questions are:

1) Given the PC spec listed below will WinXP 32 work or will I have to use WinXP 64?
2) Am I likely to find other issues with hardware compatibility? (I believe USB3 needs W7 64bit to run? But currently I have no actual use for USB3)
3) If I install WinXP 64 am I likely to find compatibility issues with programs that worked on WinXP 32bit?
4) I have read that 32bit OS will not make any real use of above 3GB ram - is this true?

I use my PC for many things but mainly Internet browsing (of course) gaming (mainly RTS but some GTA-type games from time to time) photo editing and video processing and for watching downloaded films & TV shows. Nothing too impressive or demanding.

The answer I hope to get is: "yeah, you can fresh install Windows XP SP2 32bit but you'll not be able to use USB3 and the increase in memory will not be fully utilised" or something along those lines.

Apologies for long post but I wanted to gat as much info across as possible12

Thanks in advance!

AMD PHENOM II 965 QUAD CORE 3.4GHZ CPU (6MB CACHE)
GIGABYTE 78LMT-USB3 MOTHERBOARD
8GB DDR3 1600MHZ FAST MEMORY (16GB MAX)
ATI HD 6570 2GB GRAPHICS CARD WITH HYPER MEMORY TO 3.8GB
2TB SATA HARD DRIVE
24 X SATA DVDRW WRITER
6 x USB 2.0 PORTS (4 x REAR + 2 AT FRONT / SIDE OF FACIA) +FRONT USB 3.0 PORT
ONBOARD 5.1 SOUND CARD + REAR & FRONT AUDIO INPUTS AND OUTPUTS