I would not recommend doing ANY gaming on the HD 5450 with only a 220w PSU.
Here are two reviews from respected sites, one showing that the system uses 201w with no load and 232w under load and the second just reports the minimun requirement of 300w.
http://www.tweaktown.com/reviews/3120/sapphire_radeon_hd_5450_512mb_gddr3_video_card/index13.html
http://www.guru3d.com/article/radeon-5450-review/6
Note that the second also specifies the requirement of at least 30 amps on the 12v rail, which itself is 360w.
I generally try to run a PSU at 60% to 70% of load - which means even for a 200w idle system at 70% load requires a 285w PSU. I might streach that a little in a real tight situation, but not without some trepidation and not without a GOOD quality PSU. Store bought systems usually do not have a good quality PSU. Do you know what the make and model of your system is?
The non-gaming page at upgradevideocards.com has information and recommendations about HTPC system video cards. You might want to review what they have, but nothing there is good for a 220w system.
http://www.upgradevideocards.com/nongaming.html
The only option I see is the GT 210 which uses 164w in idle and 186w with a full load. At 186w that would be an 85% load on your PSU, assuming it is capable of delivering the full 220w - many lower quality PSUs do not. See this review for power consumption and, if interested, read the rest of the review so you are familiar with its poor gaming performance:
http://www.guru3d.com/article/inno3d-geforce-gt-210-and-220-review/4
Note that page states that the outcome of over taxing the PSU can be:
bad 3D performance
crashing games
spontaneous reset or imminent shutdown of the PC
freezing during gameplay
PSU overload can cause it to break down
I wish I could be more positive, but I don't want you to buy a new card and run into problems.
Have you considered upgrading the PSU? Either of these would be a good option if they fit in your budget and in your case:
Antec Earthwatts 380w - $48 with shipping
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371033
Corsair Builder Series 430w - $23 after rebate and with shipping
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139017
It would be nice if you could stretch to this one, and have capacity for a real good video card should you decide for a better upgrade:
Corsair Builder series - 600w - $51 after rebate and with shipping
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139019
However, since I don't know what CPU you have I have no idea if it can handle a stronger card.