~$300 to GPU/PSU/RAM for Dell Vostro 470

I started off thinking I would just need a video card an ram, but as was reading about upgrading video cards, I read that the 350W might not be enough depending on what video card I get. Then it lead to the fact that some video cards are longer than others so it may not fit the case correctly or there may not be enough plugs from the PSU for the video card if it needed it?

On the invoice it just lists "4GB DDR3 SDRAM 1600MHz Vos" but when it comes I'll definitely take a look. I figured I might as well go 16GB, to accommodate for the future, unless it's better value for the money if I just go with 8GB right now?

Any recommendations for video cards? I've read that some people used the HD 7850? But when I look into these, there are several brands that make that, is there a specific one that I should use? Or does it not matter?

Thanks for the suggestions! So I just received my tower from Dell today... didn't come with extra HDD brackets so called in CS and they're sending an extra HDD bracket, which may not be good enough considering I need a bracket for a SSD. There are 4 SATA connectors and I assume the SATA 3 and SATA 6 connections depend on the drives and not to mobo connections? (Please correct me if I'm wrong) I'm hoping to do a SSD (I got the Samsung 840 120GB that just went on sale for ~$90 after taxes and due to recommendation from RFD post http://forums.redflagdeals.com/upgrading-dell-vostro-470-a-1325575/) for my OS/applications, HDD for source files, and HDD for final project files (workflow for video editing).

Dell CS said (and the manual for the Vostro 470) said that, "Internally accessible two 3.5-inch drive bays for SATA hard drives one mSATA SSD drive (optional)", and the CS said it was due to the PSU limitations, since I asked if I could add more than 2 drives since there's 4 SATA connections... but another CS rep said that it's not recommended to upgrade the PSU? And said it was because of mobo/processor compatibility, is this correct? (I didn't think they mattered)

But if I can upgrade the PSU, I'm looking at the Corsair CX500 (the XFX were a bit too expensive and other Corsair models, trying to keep it under $100). The title of it says ATX, which I know is associated with the mobo, but I'm not sure if the mobo on the Vostro 470 is an ATX, as AndrewRFD posts on the original thread (http://forums.redflagdeals.com/dell-vostro-470-desktop-w-core-i7-3770-4gb-ram-500gb-hd-geforce-gt-620-599-hot-1323240/) I saw the deal from that it's an mATX (he also says, "That being said, I have a Vostro 470 (i5-3450) where I swapped in a 750w power supply and a Radeon 6850. It's perfectly fine for my needs." which defeats what the CS rep said about upgrading PSU). Will the CX500 still work/fit?

As for video card, the AMD 100-505649 FirePro V4900 1GB 128-bit GDDR5 PCI Express 2.1 x16 Workstation Video Card reviews make it sound like a good card to go with. Is this a significant upgrade from the GT 620 that it's packaged with?

As for the case, I don't think I'll need to upgrade it, considering the manual for the Vostro 470 states that "one PCI Express x16, single-width, full length graphics card", so it sounds like it'll fit any size? The manual also states, "upto 2 GB discrete video memory", which the CS rep said that's what the mobo can handle... is that also true? I'm not planning on going all out and getting a 4GB GPU, as seen from the card I plan to get (unless otherwise told), but I'd like to know that I have the option to upgrade.

I've been reading into the FirePro and it limits the ability to play games (I know you suggested it based on my needs/budget) but I've been thinking I might be playing a few games here and there, so I looked around for the Asus GTX 660 Ti or the Asus GTX 660. It's probably that I'm not familiar with all this, but it looks like the cards are about the same, what am I paying $100 more for in the Ti version? I did read that the Ti version is best value for the dollar though, but if it's not that significant of an upgrade, I'd just go with the version without Ti.

I figured I'd bump the budget up to ~$600 now, I plan on using this computer for the long term so I might as well invest in it, making the total of the system ~$1300.

I've finalized the upgrades needed to be:
Video card - ~$300
RAM - ~$100
PSU - ~$70 (depending on video card)
SSD - $90, bought but might return based on another posters suggestion that the Samsung 840 120GB may not be the best for my needs

Thanks axehead15 for all the help! In the end, thanks to some other suggestions, I realized continuing with the 470 would've been a greater loss for me so I called in to return the machine. First time returning something to Dell and their service is great, I was at no loss trying this route out (assuming the computer checks out in the return inspection!)

I decided to just custom build it with the following: http://ca.pcpartpicker.com/p/Sbwi

Everything was bought, with the best sale in mind, and I hope it satisfies my editing and gaming needs!