So I opened up my computer (HP AMD Athlon 64 X2 Dual Core Processor 4000+) and found this power supply (no 6 pin connector).
Model: PS-5301-08HA ROHS
DC Output: 300W
Combined Power On: +5V And 3.3V Rails Shall Not Exceed 175W Max
Combined Power On: +12V And +5V Rails Shall Not Exceed 268W Max
Continuous Total DC Output Shall Not Exceed 300W
AC Input: 100-127V - 6.0A
50/60Hz 200-240V - 3.0A
So what power supply should I get to support the addition of the Radeon HD 4850?
The new 5750s (which perform equal to a 4850) use less power than a 4830 so you can get that. (it costs more though and requires 1 PCIe connector, which you can use a molex adapter for) A green version of the 5750 that doesn't need a power connector is also coming out.
If you want to upgrade your PSU as well, then yes a good 450watt should do.
Brands are an iffy thing when it comes to PSU's. For example Antec's Signature (SG) series might be the cream of the crop but it's made by Delta Electronics. So it can't be compared with their TP, Neo, or EW series as they are made by yet another "lower tier" vendor. PC Power and Cooling was major player for many years but since bought by OCZ, things have ... shall we say "changed" a bit as they try and reach a wider audience.
What do you want from a PSU ? Most would say 1) stable power and 2) quietness. To address those issues, my 1st stop is the Editor's Choice listing here:
I recommend the EC PSU's in cases where 2 GFX cards are used and the user intends more than a modest > 25% OC. However, the author of this site writes that, in his opinion, the " The [Ante] CP-850's performance makes me ponder again whether the best PSUs, going back even a couple of years, aren't already much better than they need to be. Is there really any benefit to having 1% tolerance for the 12V line? Or 20mv ripple instead of 120mv?)
So while the performance of these EC PSU's does put them in a premier "tier" of quality, do we really need that level of quality to get the job done ? So, that being said, if budget doesn't allow for a PSU off the EC PSU list, I doubt you'd be hampered by any PSU on the "Recommended List"
As for size, use any of the web based calculators to do the math for you. Antec's is pretty detailed and you don't have to d/l and install it.