Nikolay Savov :
I sad NONE - coz is Not possible if you don`t change you psu
SAPPHIRE HD 7750 1GB GDDR5 LOW PROFILE - will fit inside the case BUT
"400 Watt Power Supply is required " - system Requirements for the VGA Card
OK, I will try to be very polite here. This post will explain to unbelievers the PCIE power management.
First of all look at reviews of Sapphire 7750 low profile
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202002 On the first page of "FEEDBACK" look for 2 reviewers: TazarZero and Kyfeam.
TazarZero stated that this card works on 165 watt PSU and Kyfeam stated that it works on 240 watt PSU!
Next, for educational purposes only look at wikipedia PCIE explanation
http://en.wikipedia.org/wiki/PCI_Express#Power
It is stated there under POWER: "They can use up to 75 W (3.3 V/3 A + 12 V/5.5 A)" - this mean to regular user that PCIE maximum produced power is only 75 watt. At the end of POWER paragraph one can see this statement "Optional connectors add 75 W (6-pin) and/or 150 W (8-pin) power for up to 525 W total (75 W + 3×150 W)".
Now lets see the additional power connector visually.
This is Sapphire 7750 low profile below
Look at RIGHT UPPER CORNER, do you see any additional power connectors, I don't - there is none.
In comparison lets look at any other, more power hungry card, for an example MSI 650 TI PE, this is full size card I have chosen just to illustrate the additional 6 pin power connector
And finally lets see another source, the comparison of Radeon 7750 and Radeon 6670 cards http://www.hwcompare.com/11733/radeon-hd-6670-oem-1gb-vs-radeon-hd-7750/ look at first power comparison,
you can clearly see that power consumption of 7750 is only 55 watt.
Now lets take a look on your power supply suggestion Nikolay, the picture below shows power connectors of this PSU, pay attention to upper right corner picture
This is additional PCIE 6 pin power connector.
I wonder where you can attach it on Sapphire 7750 low profile? Show me, and I will believe you, otherwise, don't follow blindly manufacturer statements
And final not so polite statement - know your material before making uneducated statements! People coming here to get advice, not a guess!
Coming back to original question. Sapphire 7750 low profile is the only easy upgrade available for this situation, AVOX cost a fortune and require power supply upgrade.