Every little bit helps, but as MasterAuron says, the only way to get heaps of extra power is by spending money. Because you have the 1.4, you are behind the eight ball to begin with. If you sold it, and put about 2000AUD (whatever that works out to in your currency) on top of what you get for the 1.4, you might be able to get a GTi (2Lt - 103kW) another 1000AUD and you could get a GTi 180 (130kW). On the other hand if you spend 3000AUD on the 1.4 it's gonna be hard to get 130kW out of it. The 1.4 produces something like 65kW. 130 is a big jump.
The advantages of a GTi over the 1.4Lt are significantly more power, 16in (17in for the 180) alloy wheels, MUCH better suspension, and a few other items like sports seats, sports steering wheel and sports styled pedals, gear change and fuel filler cover. The last items make no improvement in performance but if you are considering adding them to the 1.4, then selling it and getting a GTi instread and they get thrown in without you having to pay for them.
On the other hand if you just want to give the 1.4 a little more poke, to make it more fun to drive, then for a 1200-1700 (AUD - less in the US, parts are cheaper even after the exchange rate), you could do the basics:
Hi-performance panel air filter and rerouted air intake from front air dam - 3kW
Headers and 2in stainless steel exhaust with high flow cat and muffler - 11kW
That takes your 1.4 to about 80kW, which will improve driveability quite a lot.
Again as MasterAuron says, weight loss will make a big difference, and is the most affordable performance mod. Trouble is, what can you throw away? This depends how much you are prepared to sacrifice the practicality and comfort of a daily driver. Back seats (check the manual for how to remove the seatbelts, if they have pre-tensioners then there will be pyrotechnics devices involved), stereo including speakers, spare wheel, carpet and underlay, rear windscreen wiper and motor, doorcards can all go.
But the bottom line is you will get much more bang for your buck by selling the 1.4 and upgrading.