[Solved] Cheap Ugrades for HP Media Center

So, we have an HP Media Center m7160n from 2005, and I need to have it last me for about three more years until I get a new computer for college. It'll be replacing my Acer Extensa 4420 laptop as my primary computer because the laptop's hinge is broken and the stupid thing keeps overheating and crashing. I'll list some of the components in the desktop...
-Pentium D 820 2.8GHz@3.2GHz
-Intel GMA 950 Integrated Graphics
-Conexant Falcon II Tuner
-300GB 7200rpm HDD
-LightScribe DVD burner
-DVD drive
-HP expansion bay (portable HDD)
-Multi-card reader
-300W PSU
-3GB (2x512MB, 2x1GB) of DDR2 RAM
-Asus P5LP-LE (Limestone) Motherboard

It currently runs on XP MCE, I'm wanting to switch to Win7 Ultimate (already have 2 keys)

I was looking at a Radeon HD4670 for a graphics card, but somebody took the last one on amazon and the price went from $50 to $75, but I should be fine with something cheaper. I was also looking at a Pentium D 840. I know that it's really old, but it's slightly faster, works on the mobo, and costs less than $30 now. I don't need something uber-powerful, so I'll just list out some common things that I do on the computer:
-Use multiple monitors (need a graphics card for this)
-Watch a movie (laptop sadly can't do HD well at all, I'd like to watch AVCHD on the desktop)
-Type a document in Word
-Make a PowerPoint
-Edit video (Using MAGIX Movie Edit Pro Plus 17 DLV)
-Make a Flash animation (Adobe Flash Profesional CS5, may upgrade to 5.5 when I can)
-Burn a DVD (ConvertXtoDVD, Corel DVD MovieFactory, Movie Edit Pro Plus)
-Burn an AVCHD disc (Movie Edit Pro Plus, Adobe Media Encoder+tsmuxer+IMGburn)
-Capture DVI Video via FireWire
-Stream in Procaster while making a Flash animation
-Play a videogame (around PS2-era, not too demanding at all)

As you can see, I don't do things that are too demanding, but I would still like to have decent performance. The outdated Pentium D 820 already runs in circles around my laptop's AMD Athlon 64 X2 TK-57 at video editing and such. I'm used to a pretty slow computer, so I shouldn't have too hard of a time making it through senior year with something that's not very powerful. Anyways, based on what I do, can you recommend some *cheap* parts for me?
Some stuff I definitely need:
-A video card for dual-monitors, HD playback, and light gaming/video editing(needs to work w/ PCI-e v1, unless there's a cheap mobo replacement to overcome that) I was looking at the HD5650 now that the HD4670 went up in price.
-A wifi adapter (preferably Conventional PCI, if not, USB is fine)

Thanks in advance for any help!

EDIT: Oh, and if at all possible, I'd like a little extra room for upgrades in the future; once I have a job, I'll probably upgrade the CPU and mobo so that they are more current at that time.

Best answer selected by halomademeapc.

On video cards, HD6670 is known to work with older pcie 1.x boards and is both much stronger than HD4650 and requires no external connector so will work with your current PSU. Cost $90. Google it.