What can i do?

You will need a new board for the amd 255, and probably new ram. I believe your old board uses ddr; newer boards take ddr2 or ddr3. Get a retail boxed 255, and it comes with a heatsink and three year warranty. Newegg may have a combo special with board for around $100; ddr3 2x1 gb kits will run around $40. Your power supply may work fine; you can use a 20 pin connector with 24 pin atx port on your new board; it only fits one way, leaving 4 pins on one side open. For a cheaper alternative, frys in stores only has the Intel e3300 dual core for $37.99 retail boxed and an msi g31 board that uses ddr2 for only $29.99 after rebate; both deals are good through Wed this week.

1. "old serial ata" - I'm guessing you're referring to PATA (parallel ATA). This is the old technology for connecting drives. Not all boards have this because the new tech is SATA.

2. I'd recommend a higher wattage PSU. If you can afford it, try buying one that will carry you through future upgrades/addons. A 250 W PSU will be enough to power up the CPU and mobo, but you could result in underpowering the mobo when you start to connect more devices. The typical result of underpowering would be a power on with no display.

3. Athlon II X2 + any 1 GB GPU should have at least 350 (400 W would be better) W with 2+ 12V rails (1 for mobo; one for GPU). For best results, use an online PSU calculator like this one: http://www.corsair.com/psufinder/default.aspx

4. Are you referring to the stock heatsink that is on your P4 or an aftermarket? Aftermarket CPU cooler/heatsinks have various applications, but I don't recall any that fit both the P4 and A2X2. A good place to check would be www.frozencpu.com

First, you CANNOT use an AMD CPU in an Intel MOBO <nor> an Intel CPU in an AMD MOBO. I answered before noticing all of the 'real problems' - the technology is VERY different now, and while I 'get' the needs there are brick walls with some of your choices that need to be discussed.

ANS 1 - Time for a new HDD; don't make decisions based upon an OLD HDD with one foot on the banana peal. Otherwise, you are very limited.
ANS 2 - 250W with an Integrated GPU and no OC is barely okay, but with an avg GPU {e.g. HD 5770} then 350W minimum.
ANS 3 - You cannot transplant an AGP GPU into a PCIe MOBO; so you'll need a new GPU.
ANS 4 - Some fans have kits and can be reused with different sockets.

Cheap solution - $545 - http://www.cyberpowerpc.com/system/Back_To_School_2010/

"First, you CANNOT use an AMD CPU in an Intel MOBO <nor> an Intel CPU in an AMD MOBO" obviously...

New HDD is not a problem.
Okay I will look into, say a 400W PSU.
Don't suppose the wiring from old CD/DVD internal drives have changed since 2004 have they?
Its a stranded fan + heatsick but i know were to get a good new one  

Also i live in the UK  

http://www.ebuyer.com/product/142399 < RAM
http://www.ebuyer.com/product/221178 < MOBO
http://www.ebuyer.com/product/176157 < FAN
http://www.ebuyer.com/product/237554 < CPU*
http://www.ebuyer.com/product/174678 < GPU**

*I seen the AMD Athlon 64 X2 7850, its cheaper and dose a better job by some of the reviews i have seen. I don't need a future proof machine, just a ok one for 2 years then it will get scrapped  

**Any good for... lets say, Fallout: NV on 726/720 at medium?

UK http://www.pcspecialist.co.uk/

£438
Case COOLERMASTER ELITE 310 BLUE CASE
Processor (CPU) AMD ATHLON II X4 645 (3.10GHz/2MB CACHE/AM3)
Motherboard ASUS® M4A87TD/USB3: DUAL DDR3,SATA 6.0GB/s, USB 3.0
Memory (RAM) 4GB SAMSUNG DDR3 DUAL-DDR3 1333MHz (2 X 2GB)
Graphics Card 1GB ATI RADEON™ HD 5450 PCI EXPRESS - DirectX® 11
2nd Graphics Card NONE
Memory - 1st Hard Disk 500GB SERIAL ATA 3-Gb/s HARD DRIVE WITH 8MB CACHE (7,200rpm)
2nd Hard Disk NONE
RAID NONE
1st DVD/BLU-RAY Drive 24x DUAL LAYER DVD WRITER ±R/±RW/RAM
2nd DVD/BLU-RAY Drive NONE
Memory Card Reader INTERNAL 52 IN 1 CARD READER (XD, MS, CF, SD, etc) + 1 x USB 2.0 PORT
Power Supply & Case Cooling 450W Quiet 80 PLUS Dual Rail PSU + 120mm Case Fan (£29)
Processor Cooling SUPER QUIET 22dBA TRIPLE COPPER HEATPIPE CPU COOLER (£19)
Sound Card ONBOARD 8 CHANNEL (7.1) HIGH DEF AUDIO (AS STANDARD)
Network Facilities ONBOARD 10/100/1000 GIGABIT LAN PORT
USB Options 6 x USB 2.0 PORTS @ BACK PANEL (MIN 2 FRONT PORTS) AS STANDARD
Modem NONE, I WILL BE USING BROADBAND
Floppy Disk Drive NONE
Firewire & Video Editing NONE
TV Card NONE
Operating System Genuine Windows 7 Home Premium 64 Bit - inc DVD & Licence (£79)
Office Software FREE Microsoft® Office Starter 2010 (Limited functionality Word & Excel)
Anti-Virus BULLGUARD INTERNET SECURITY - FREE 90 DAY TRIAL

I don't know what this is but that's not what I am asking
 

It was a polite way of saying what you posted - you'll regret Building. It's all horribly outdated and is two steps forward and one backward. Super-cheap rigs are not my 'thing' and I would try the New Build section as linked below.  

By the time you factor in a new PSU + HDD the total cost is ~£280+ vs £438 = $158 ; then subtract off DVD & Case.

Otherwise I would post it in the 'New Build' section - http://www.tomshardware.com/forum/forum-31-322.html

lol sorry. ok thanks for the help anyways  

What everybody is trying to tell you is that you are wasting money trying to upgrade such an old system.

No, what I am saying I don't build "lower cost" rigs and don't have a good feel for recommending other than 'guessing' how it will all work together. The OP is looking at a DDR3/DDR2 MOBO which I know is bad; you can use one or the other but not both.

Therefore, you {JSC} can help! Good Luck!