Graphic card and PSU compatibility issue

Hello,
My System is a HP-Compaq dc7800 (tower) with the following specs,

CPU: Intel Core2Duo E6750 @2.66Ghz
MOBO: dont know- is a BTX Form Factor
Memory: 3GB
Onboard Graphics: Intel Q35 express/ Intel GMA3100 (very slow)
PSU: HP 365W


Now here i wanted to buy a Graphics card for my system and im having issues regarding (1)power supply (2) PCI-e 2.1/3.0 support
(3) Motherboard being BTX Form Factor

(1)Power supply: i wanted to buy an XFX HD 6770 which requires 500W PSU(minimum) (max TDP is 108W of the card) and i have 365W so then i thought to buy a HIS HD 7750 which is a 28nm design card and requires less power (max TDP 55W) than HD 6770(Max TDP 108W) but with a performance letdown BUT i searched the HIS website and it said it requires 400W PSU and i have 365W so will it work on my PSU?
secondly i also planned to buy a new PSU like Cooler Master 500W but will the new PSU be compatible with my system having BTX form factor of HP branded casing?? if no then what should i do?



(2) PCI-e 2.1/3.0 support issue: my model of PCI-e is 1.0 or 1.1 i think but the radeon HD 6770 is 2.1 and HD 7750 is 3.0 so will they be perfectly compatible with my mobo.. i know that they are backwards compatible with performance decrease but what if i buy the card and it doesnt work plz help me out


(3) Motherboard being BTX Form Factor: as we all know m-boards are installed on the right side of the casing but my HP system is opposite its is on the left side which quite freaks me out regarding compatibiliy : will i be able to install the new PSU Cooler Master 500W? and will the graphic card be installed on this card UPSIDE DOWN?  

Looking forward to your helpful and technical reply
Regards: Anas Shaikh